The role of skin tests with polyethylene glycol and polysorbate 80 in the vaccination campaign for COVID-19: results from an Italian multicenter survey.

Summary: Background. International guidelines suggested skin tests with Polyethylene-glycol (PEG) and polysorbate 80 (PS-80), to investigate a possible hypersensitivity to these excipients either to identify subjects at risk of developing allergic reactions to Covid-19 vaccines, or in patients with suspected IgE mediated hypersensitivity reactions (HR) to the Covid-19 vaccine. The main purpose of this study was to investigate the prevalence of PEG and PS sensitization in patients with a clinical history of HR to drugs containing PEG/PS and in patients with a suspected Covid-19 vaccine immediate HR. Methods. This was a multicenter retrospective study conducted by allergists belonging to 20 Italian medical centers. Skin testing was performed in 531 patients with either a clinical history of suspected hypersensitivity reaction (HR) to drugs containing PEG and/or PS-80 (group 1:362 patient) or a suspected HR to Covid-19 vaccines (group 2: 169 patient), as suggested by the AAIITO/SIAAIC guidelines for the "management of patients at risk of allergic reactions to Covid-19 vaccines" [1]. Results. 10/362 (0.02%) had positive skin test to one or both excipients in group 1, 12/169 (7.1%) in group 2 (p less than 0.01). In group 2 HRs to Covid-19 vaccines were immediate in 10/12 of cases and anaphylaxis occurred in 4/12 of patients. Conclusions. The positivity of skin test with PEG and or PS before vaccination is extremely rare and mostly replaceable by an accurate clinical history. Sensitization to PEG and PS has to be investigated in patients with a previous immediate HR to a Covid-19 vaccine, in particular in patients with anaphylaxis.

Delayed diagnosed intermuscular lipoma causing a posterior interosseous nerve palsy in a patient with cervical spondylosis: the "priceless" value of the clinical examination in the technological era.

BACKGROUND: Posterior interosseous nerve (PIN) palsy may present with various symptoms, and may resemble cervical spondylosis. CASE REPORT: We report about a 59-year-old patient with cervical spondylosis which delayed the diagnosis of posterior interosseous nerve (PIN) palsy due to an intermuscular lipoma. Initial right hand paraesthesias and clumsiness, together with MR findings of right C5-C6 and C6-C7 foraminal stenosis, misled the diagnostic investigation. The progressive loss of extension of all right hand fingers brought to detect a painless mass compressing the PIN. Electrophysiological studies confirmed a right radial motor neuropathy at the level of the forearm. RESULTS: Surgical tumor removal and nerve decompression resulted in a gradual motor deficits recovery. CONCLUSIONS: A thorough clinical examination is paramount, and electrophysiology may differentiate between cervical and peripheral nerve lesions. Ultrasonography and MR offer an effective evaluation of lipomas, which represent a rare cause of PIN palsy. Surgical decompression and lipoma removal generally determine excellent prognoses, with very few recurrences.

Cross-reactivity and Tolerability of Ertapenem in Patients With IgE-Mediated Hypersensitivity to ß-Lactams.

BACKGROUND AND OBJECTIVE: Administration of carbapenems to ß-lactam-allergic patients has always been considered potentially harmful because of a 47.4% rate of cross-reactivity to imipenem reported in a single study. Nevertheless, recent studies have shown that the rate of cross-reactivity of imipenem and meropenem with penicillins is lower than 1%. The aim of this study was to evaluate the possibility of using ertapenem in patients with an established IgE-mediated ß-lactam allergy. PATIENTS AND METHODS: We studied all participants who came to our allergy unit and had a clinical history of immediate hypersensitivity reactions to ß-lactams. The inclusion criteria were a positive skin test result to at least 1 ß-lactam molecule and/or positive specific IgE (when available). All participants underwent immediate-type skin tests with several ß-lactam molecules including ertapenem. Challenges with intravenous ertapenem were performed on 2 different days in patients with negative skin test results. RESULTS: We examined 49 patients with a clinical history of immediate reactions to ß-lactams. All the patients had positive skin tests and/or positive specific IgE to at least 1 ß-lactam reagent and negative carbapenem skin tests. Thirty-six patients agreed to undergo the challenges and 35 tolerated the full dose of ertapenem. CONCLUSIONS: The practice of avoiding carbapenems in patients with ß-lactam allergy should be abandoned considering the very low rate of cross-reactivity. ß-Lactam-allergic patients who need ertapenem therapy should undergo skin tests and, if negative, a graded challenge to assess tolerability.

Cross-reactivity and tolerability of Ertapenem in patients with IgE-mediated hypersensitivy to Beta-lactams

Background and Objective: Administration of carbapenems to β-lactam-allergic patients has always been considered potentially harmful because of a 47.4% rate of cross-reactivity to imipenem reported in a single study. Nevertheless, recent studies have shown that the rate of cross-reactivity of imipenem and meropenem with penicillins is lower than 1%. The aim of this study was to evaluate the possibility of using ertapenem in patients with an established IgE-mediated β-lactam allergy. Patients and Methods: We studied all participants who came to our allergy unit and had a clinical history of immediate hypersensitivity reactions to β-lactams. The inclusion criteria were a positive skin test result to at least 1 β-lactam molecule and/or positive specific IgE (when available). All participants underwent immediate-type skin tests with several β-lactam molecules including ertapenem. Challenges with intravenous ertapenem were performed on 2 different days in patients with negative skin test results. Results: We examined 49 patients with a clinical history of immediate reactions to β-lactams. All the patients had positive skin tests and/or positive specific IgE to at least 1 β-lactam reagent and negative carbapenem skin tests. Thirty-six patients agreed to undergo the challenges and 35 tolerated the full dose of ertapenem. Conclusions: The practice of avoiding carbapenems in patients with β-lactam allergy should be abandoned considering the very low rate of cross-reactivity. β-Lactam-allergic patients who need ertapenem therapy should undergo skin tests and, if negative, a graded challenge to assess tolerability (AU)

Introducción y Objetivo: Siempre se ha considerado peligrosa la administración de carbapenems a pacientes alérgicos a betalactámicos por la presencia de reactividad cruzada en el 47,4% de los casos descrita en un estudio previo. Sin embargo, estudios recientes han mostrado que la reactividad cruzada de imipenem y meropenem con penicilinas es inferior al 1%. El objetivo de este estudio es valorar el uso de ertapenem en pacientes diagnosticado de alergia IgE mediada a betalactámicos. Pacientes y Métodos: Se incluyeron todos los pacientes que acudieron a nuestra unidad de Alergia con historia clínica de alergia inmediata a betalactámicos. Los criterios de inclusión fueron prueba cutánea positiva con al menos un betalactámico y/o IgE específica positiva (cuando estuviese disponible). Se realizaron pruebas cutáneas con betalactámicos, incluyendo ertapenem, con lectura inmediata en todos los pacientes. Se realizaron pruebas de provocación endovenosas con ertapenem en los pacientes con pruebas cutáneas negativas frente al mismo en dos días diferentes. Resultados: Se incluyeron 49 pacientes con historia clínica de alergia inmediata a betalactámicos. Todos los pacientes tenían pruebas cutáneas positivas y/o IgE específica positiva al menos a uno de los betalactámicos así como prueba cutánea negativa con carbapenémicos. Treinta y seis pacientes aceptaron la realización de pruebas de provocación con ertapenem que fueron tolerados por treinta y cinco de dichos pacientes. Conclusión: El hecho de recomendar evitar carbapenems en pacientes con alergia a betalactámicos debería ser abandonado, dada la baja reactividad cruzada que presentan. En los pacientes con alergia a betalactámicos que necesiten ertapenem se deberían realizar pruebas cutáneas con el fármaco y en caso de ser negativas, realizar un test de exposición progresiva para confirmar su tolerancia (AU)

Neurological outcome in a series of 58 patients operated for traumatic thoracolumbar spinal cord injuries.

BACKGROUND: Traumatic thoracolumbar spinal fractures represent approximately 65% of all traumatic spinal fractures and are frequently associated to permanent disability with significant social and economic impact. These injuries create severe physical limitations depending on neurological status, level of fracture, severity of injury, patient age and comorbidities. Predicting neurological improvement in patients with traumatic spinal cord injuries (SCIs) is very difficult because it is related to different preoperative prognostic factors. We evaluated the neurological improvement related to the preoperative neurological conditions and the anatomic level of spinal cord injury. METHODS: From January 2004 to June 2010, we operated 207 patients for unstable thoracolumbar spinal fractures. We carried out a retrospective analysis of 69 patients with traumatic SCIs operated on by a posterior fixation performed within 24 hours from the trauma. The preoperative neurological conditions (ASIA grade), the type of the fracture, the anatomic level of spinal cord injury and the postoperative neurological improvement were evaluated for each patient. RESULTS: The ASIA grade at admission (P = 0,0005), the fracture type according to the AO spine classification (P = 0,0002), and the anatomic location of the injury (P = 0,0213) represented predictive factors of neurological improvement at univariate analysis. The preoperative neurological status (P = 0,0491) and the fracture type (P = 0,049) confirmed a positive predictive value also in the multivariate analysis. CONCLUSIONS: Our study confirms that the preoperative neurological status, the fracture type and the anatomic location of the fracture are predictive factors of the neurological outcome in patients with spinal cord injury.

Inter-ethnic differences in lymphocyte sensitivity to glucocorticoids reflect variation in transcriptional response.

Glucocorticoids (GCs) are steroid hormones widely used as pharmaceutical interventions, which act mainly by regulating gene expression levels. A large fraction of patients (∼30%), especially those of African descent, show a weak response to treatment. To interrogate the contribution of variable transcriptional response to inter-ethnic differences, we measured in vitro lymphocyte GC sensitivity (LGS) and transcriptome-wide response to GCs in peripheral blood mononuclear cells from African-American (AA) and European-American (EA) healthy donors. We found that transcriptional response after 8 h treatment was significantly correlated with variation in LGS within and between populations. We found that NFKB1, a gene previously found to predict LGS within populations, was more strongly downregulated in EAs on average. NFKB1 could not completely explain population differences, however, and we found an additional 177 genes with population differences in the average log2 fold change (false discovery rate<0.05), most of which also showed a weaker transcriptional response in AAs. These results suggest that inter-ethnic differences in GC sensitivity reflect variation in transcriptional response at many genes, including regulators with large effects (for example, NFKB1) and numerous other genes with smaller effects.

Acute tension pneumocephalus secondary to whole spine pneumorrhachis as an unusual presentation of a colon cancer complicated by a transsacral cerebrospinal fluid leak.

A 52-year-old woman who was having chemotherapy for treatment of an adenocarcinoma of the colon, was admitted to the emergency department because of a moderate neurological impairment. Head CT scan showed bifrontal pneumocephalus without fractures or discontinuities of the skull base. A few hours later, following the patient's neurologic deterioration, a new CT scan showed tension pneumocephalus with air diffusion throughout the posterior cranial fossa and cervical spine. Because of air bubbles into the cervical spine, an MRI of the entire spinal canal was done. This exam revealed a whole spine pneumorrhachis along with a transforaminal air passage through the first right sacral foramen and a pyogenic collection anterior to sacral bone. An abdomen CT scan showed a massive relapse of the colon cancer and confirmed a hypodense collection contiguous to the anterior sacral surface, causing erosion of the sacral bone and dural layer with air penetration into the spinal canal. Neurosurgical treatment by a lumbosacral laminectomy and duraplasty was followed by tumour removal and omental covering of the pelvis. Her neurological symptoms were resolved completely. One month later, the patient began adjuvant chemotherapy.

SNP discovery, expression and cis-regulatory variation in the UGT2B genes.

UGT2B enzymes metabolize multiple endogenous and exogenous molecules, including steroid hormones and clinical drugs. However, little is known about the inter-individual variation in gene expression and its determinants. We re-sequenced candidate regulatory regions and the partial coding regions (41.1 kb) of UGT2B genes and identified 332 genetic variants. We measured gene expression in normal breast and liver samples and observed different patterns. The expression levels varied greatly across individuals in both tissues and were significantly correlated with each other in liver. Genotyping of tagging single-nucleotide polymorphisms (SNPs) in the same samples and association tests between genotype and transcript levels identified 62 variants that were associated with at least one UGT2B mRNA levels in either tissue. Most of these cis-regulatory SNPs were not shared between tissues, suggesting that this gene family is regulated in a tissue-specific manner. Our results provide insight into studying the role of UGT2B variation in hormone-dependent cancers and drug response.

Evolutionary adaptations to dietary changes.

Through cultural innovation and changes in habitat and ecology, there have been a number of major dietary shifts in human evolution, including meat eating, cooking, and those associated with plant and animal domestication. The identification of signatures of adaptations to such dietary changes in the genome of extant primates (including humans) may shed light not only on the evolutionary history of our species, but also on the mechanisms that underlie common metabolic diseases in modern human populations. In this review, we provide a brief overview of the major dietary shifts that occurred during hominin evolution, and we discuss the methods and approaches used to identify signals of natural selection in patterns of sequence variation. We then review the results of studies aimed at detecting the genetic loci that played a major role in dietary adaptations and conclude by outlining the potential of future studies in this area.

Prediction of CYP3A4 enzyme activity using haplotype tag SNPs in African Americans.

The CYP3A locus encodes hepatic enzymes that metabolize many clinically used drugs. However, there is marked interindividual variability in enzyme expression and clearance of drugs metabolized by these enzymes. We utilized comparative genomics and computational prediction of transcriptional factor binding sites to evaluate regions within CYP3A that were most likely to contribute to this variation. We then used a haplotype tagging single-nucleotide polymorphisms (htSNPs) approach to evaluate the entire locus with the fewest number of maximally informative SNPs. We investigated the association between these htSNPs and in vivo CYP3A enzyme activity using a single-point IV midazolam clearance assay. We found associations between the midazolam phenotype and age, diagnosis of hypertension and one htSNP (141689) located upstream of CYP3A4. 141689 lies near the xenobiotic responsive enhancer module (XREM) regulatory region of CYP3A4. Cell-based studies show increased transcriptional activation with the minor allele at 141689, in agreement with the in vivo association study findings. This study marks the first systematic evaluation of coding and noncoding variation that may contribute to CYP3A phenotypic variability.

Decompressive craniectomy for medically refractory intracranial hypertension due to meningoencephalitis: report of three patients.

BACKGROUND: Meningoencephalitis may sometimes cause medically refractory intracranial hypertension and brain herniation. In such patients death is common. There are a limited number of reports on the use of decompressive craniectomy as a life saving measure in these circumstances with some good results. The aim of the study was to report experience in three further patients. MATERIALS AND METHODS: In a 15-month period, three patients affected by acute meningoencephalitis were surgically treated by decompressive craniectomy at the Department of Neurosurgery of the Polytechnic University of Ancona. In all patients common symptoms at presentation were headache, fever and neck rigidity, rapidly followed by the development of focal neurological deficits and coma. Intracranial pressure monitoring was always performed and correlated with serial CT scan examinations. Because of the development of severe intracranial hypertension refractory to conventional medical treatment, a decompressive hemicraniectomy was performed in two patients and a bifrontal decompressive craniectomy in the third one. Bacterial meningoencephalitis was diagnosed in two patients, viral meningoencephalitis in the remaining one. FINDINGS: One patient died 3 days after surgery. The remaining two completely recovered consciousness, with no residual focal neurological deficit. CONCLUSIONS: Surgery resulted in an immediate reduction of intracranial pressure in two of the three patients with severe meningoencephalitis. Decompressive craniectomy may be a useful option in the management of a patient with medically refractory intracranial hypertension caused by meningoencephalitis. Early intervention may enhance its benefits.

Sublingual immunotherapy: from safety to mechanism of action.

Allergen specific immunotherapy is an important option for the treatment of respiratory allergy and its clinical efficacy has been clearly demonstrated by several studies. However, the injective route of administration and the possibility of severe side effects has limited its use in children and led to the introduction of new forms of administration. Sublingual immunotherapy (SLIT) has proven to be an effective and safe treatment for respiratory allergy. However, its mechanism of action is still debated. Pharmacokinetic studies showed that, differently from nasal mucosa, allergen extracts administered by SLIT are not immediately adsorbed but are long retained before being drained to local lymph nodes. This difference may be responsible of the absence of severe side effects and instead of short-lasting local symptoms. Studies by biopsies of the oral mucosa should greatly help in defining the presence and the role of cells involved in the mechanisms of oral tolerance.

Sequence diversity and haplotype structure at the human CYP3A cluster.

The four members of the human CYP3A subfamily play important roles in the clearance of xenobiotics, hormones, and environmental compounds. Many SNPs at the CYP3A locus have been characterized, with several showing large allele frequency differences across populations. In addition to the effects of CYP3A SNPs on drug metabolism, recent studies have highlighted the potential for CYP3A variation in susceptibility to several common phenotypes, including hypertension and cancer. We previously showed that the CYP3A4 and CYP3A5 genes have a strong haplotype structure at varying frequencies across ethnic groups. Here, we extend our re-sequencing survey to the remaining CYP3A genes in the same cluster, CYP3A7 and CYP3A43. Our study identified a large number of SNPs in coding and conserved noncoding sequences, several of which are common. The combined data set allows us to investigate patterns of sequence variation and linkage disequilibrium at the entire CYP3A locus for use in future association studies.

Comparative genomics analysis of human sequence variation in the UGT1A gene cluster.

Common polymorphisms within the human UGT1A gene locus are associated with irinotecan and tranilast toxicity. To uncover additional functional variation across this gene cluster, cross-species sequence comparisons were performed. Evolutionarily conserved segments (a total of 47.1 kb) were re-sequenced in 24 African-American, 24 European-American, and 24 Asian individuals, and 381 segregating sites (including 123 singletons) were identified. Highly conserved coding sites were less likely to be polymorphic than diverged sites (P<0.0001) but this pattern was not observed at non-coding sites (P=0.1025). Among coding variants, the distribution of those computationally predicted to affect function was skewed toward low frequencies. Some alleles occurred at similar frequencies in each population; others had wide disparities. Although strong linkage disequilibrium was detected among the hepatically expressed genes, the degree of linkage disequilibrium varied among populations. These results suggest that rare functional gene variants and inter-population variability must be considered in the interpretation of association studies between UGT1A and drug metabolism/toxicity phenotypes.

Supratentorial cavernomas in eloquent brain areas: application of neuronavigation and functional MRI in operative planning.

AIM: Cavernomas located in eloquent areas of cerebral hemispheres represent a challenge for the neurosurgeon. An accurate surgical approach is essential to completely remove the lesion with function preservation. Aim of this study was to evaluate the usefulness of integration between standard magnetic resonance imaging (MRI) for neuronavigation and functional MRI (fMRI) in preoperative planning and intraoperative removal of cavernomas. METHODS: Between June 2000 and December 2002, 21 patients underwent surgery for supratentorial subcortical cavernomas. Eleven lesions were located adjacent to eloquent brain areas. All the patients in the series underwent MRI for neuronavigation and, since January 2002, in 6 cases of lesions located in eloquent areas, fMRI was also performed, with subsequent images fusion. The surgical approach was performed via the transgyral route under conventional and ultrasound-guided neuronavigation. RESULTS: All the lesions were totally removed. No morbidity was seen in patients harbouring lesions in non eloquent areas. Four patients with lesions in critical areas suffered transient focal deficits, but only one patient of this series was operated on by the auxilium of image fusion. In 7 patients operated on by conventional image-guided surgery and affected by preoperative seizures, no further seizures were observed after surgery. In 3 patients more hosting lesions neighbouring critical areas, the perilesional ring was not removed, observing persistence of seizures pharmacologically treated. In 4 of the 6 patients (all affected by seizures), operated on by fMRI auxilium, lesion removal was associated to the removal of the perilesional ring. No further epilepsy was seen in these patients. CONCLUSIONS: In all the cases the use of neuronavigation allowed minimally invasive approaches and radical excision of the lesions. Moreover, fMRI seemed to provide important additional information in patients with lesions in eloquent brain areas, allowing a more aggressive approach on the perilesional tissue to the aim of resolving seizures, in absence of an increase in the morbidity rate.

Population genetics of CAPN10 and GPR35: implications for the evolution of type 2 diabetes variants.

A positional cloning study of type 2 diabetes in Mexican Americans identified a region, termed "NIDDM1," on chromosome 2q37 with significant linkage evidence. Haplotype combinations at the calpain-10 gene (CAPN10) within this region were shown to increase diabetes risk in several populations. On the basis of the thrifty genotype hypothesis, variants that increase susceptibility to type 2 diabetes under modern lifestyle conditions provided a survival advantage in past environments by increasing the efficiency of energy use and storage. Here, our goal is to make inferences about the evolutionary forces shaping variation in genes in the NIDDM1 region and to investigate the population genetics models that may underlie the thrifty genotype hypothesis. To this end, we surveyed sequence variation in CAPN10 and in an adjacent gene, G-protein-coupled receptor 35 (GPR35), in four population samples from different ethnic groups. These data revealed two distinct deviations from the standard neutral model in CAPN10, whereas GPR35 variation was largely consistent with neutrality. CAPN10 showed a significant deficit of variation in the haplotype class defined by the derived allele at SNP44, a polymorphism that is significantly associated with diabetes in meta-analysis studies. This suggests that this haplotype class was quickly driven to high frequency by positive natural selection. Interestingly, the derived allele at SNP44 is protective against diabetes. CAPN10 also showed a local excess of polymorphism and linkage disequilibrium decay in intron 13. Simulations show that this pattern may be explained by long-standing balancing selection that maintains multiple selected alleles. Alternatively, it is possible that the local mutation and recombination rates changed since the divergence of human and chimpanzee; this scenario does not require the action of natural selection on intron 13 variation.

CYP3A variation and the evolution of salt-sensitivity variants.

Members of the cytochrome P450 3A subfamily catalyze the metabolism of endogenous substrates, environmental carcinogens, and clinically important exogenous compounds, such as prescription drugs and therapeutic agents. In particular, the CYP3A4 and CYP3A5 genes play an especially important role in pharmacogenetics, since they metabolize >50% of the drugs on the market. However, known genetic variants at these two loci are not sufficient to account for the observed phenotypic variability in drug response. We used a comparative genomics approach to identify conserved coding and noncoding regions at these genes and resequenced them in three ethnically diverse human populations. We show that remarkable interpopulation differences exist with regard to frequency spectrum and haplotype structure. The non-African samples are characterized by a marked excess of rare variants and the presence of a homogeneous group of long-range haplotypes at high frequency. The CYP3A5*1/*3 polymorphism, which is likely to influence salt and water retention and risk for salt-sensitive hypertension, was genotyped in >1,000 individuals from 52 worldwide population samples. The results reveal an unusual geographic pattern whereby the CYP3A5*3 frequency shows extreme variation across human populations and is significantly correlated with distance from the equator. Furthermore, we show that an unlinked variant, AGT M235T, previously implicated in hypertension and pre-eclampsia, exhibits a similar geographic distribution and is significantly correlated in frequency with CYP3A5*1/*3. Taken together, these results suggest that variants that influence salt homeostasis were the targets of a shared selective pressure that resulted from an environmental variable correlated with latitude.

The interhemispheric transcallosal-transversal approach to the lesions of the anterior and middle third ventricle: surgical validity and neuropsychological evaluation of the outcome.

Based on the observation of the course of callosal fibres and of their artero-venous support as appearing in a microanatomic study, the Authors propose a variant of standard callosotomy procedure by the introduction of the transverse section of callosal fibres. This technique would allow the surgeon to spare a larger number of callosal fibres by the combined effect of a lower direct mechanical traction on fibres and a lower impact on artero-venous microcircle. The neuropsychological outcome of the patients who underwent this kind of procedure was evaluated. Fourteen patients affected by occupying-space lesions involving the anterior and middle third ventricle were included in the study. Ten patients underwent transverse callosotomy, four subjects received standard sagittal callosotomy. A control group was also included in the study. All patients underwent a pre-operative and six months post-operative neuropsychological evaluation focused on performance at cognitive and attentional tasks. No disturbances in executive function were observed in either group. Patients receiving transverse callosotomy performed as well as control group subjects in attentive tasks, which is not the case of patients undergoing sagittal callosotomy who show a marked deficit in selective attention for left side visual field. The observed more favourable neuropsychological outcome supports transverse callosotomy as a valid alternative method to standard longitudinal callosotomy in third ventricle surgery.

Gene conversion and different population histories may explain the contrast between polymorphism and linkage disequilibrium levels.

To characterize linkage disequilibrium (LD) levels in human populations, we have analyzed 10 independent noncoding segments in three population samples from the major ethnic groups--that is, Africans, Asians, and Europeans. Descriptive statistics show that LD decays much faster in the African samples than in the non-African ones. With the assumption of an equilibrium model, we estimated the population crossing-over parameter (4N(e)r(bp), where N(e) is the effective population size and r(bp) is the crossing-over rate per generation between adjacent base pairs) in the presence of gene conversion. In the African sample, LD and polymorphism levels lead to similar estimates of effective population size, as expected under an equilibrium model. Conversely, in both non-African samples, LD levels suggest a smaller effective population size than that implied by polymorphism levels. This observation is paralleled by significant departures from an equilibrium model in the spectrum of allele frequencies of the non-African samples. Besides ruling out the possibility that non-African populations are at equilibrium, these results suggest different demographic history (temporal and spatial) of these groups. Interestingly, the African sample fits the expectations of an equilibrium model based on polymorphism and divergence levels and on frequency spectrum. For this sample, the estimated ratio of gene conversion to crossing-over rates is 7.3 for a mean tract length of 500 bp, suggesting that gene conversion may be more frequent than previously thought. These findings imply that disease-association studies will require a much denser map of polymorphic sites in African than in non-African populations.