Serotonin receptor 1A promoter polymorphism, rs6295, modulates human anxiety levels via altering parasympathetic nervous activity.

OBJECTIVE: The G-allele of the -1019C/G (rs6295) promoter polymorphism of the serotonin receptor 1A (HTR1A) gene has been implicated in anxiety; however, the underlying neurophysiological processes are still not fully understood. Recent evidence indicates that low parasympathetic (vagal) tone is predictive of anxiety. We thus conducted a structural equation model (SEM) to examine whether the HTR1A rs6295 variant can affect anxiety by altering parasympathetic nervous activity. METHOD: A sample of 1141 drug-free healthy Han Chinese was recruited for HTR1A genotyping. Autonomic nervous function was assessed by short-term spectral analysis of heart rate variability (HRV). Anxiety and stress levels were evaluated by the Beck Anxiety Inventory (BAI) and the Perceived Stress Scale (PSS) respectively. RESULTS: The number of the HTR1A G allele was inversely correlated with high-frequency power (HF), a parasympathetic index of HRV. The HF index was negatively associated with BAI scores. Furthermore, the good-fitting SEM, adjusting for confounding variables (e.g., age and PSS levels), revealed a significant pathway linking rs6295 variant to BAI scores via HF index modulation. CONCLUSION: These results are the first to show that HTR1A -1019C/G polymorphism influences anxiety levels by modulating parasympathetic tone, providing a neurophysiological insight into the role of HTR1A in human anxiety.

SL4, a chalcone-based compound, induces apoptosis in human cancer cells by activation of the ROS/MAPK signalling pathway.

OBJECTIVES: SL4, a chalcone-based compound, exhibits clearly inhibitory effects on HIF-1 and has been shown to effectively suppress tumour invasion and angiogenesis in vitro and in vivo. Here, studies were conducted to determine SL4's anti-apoptotic effects and its underlying mechanisms, in human cancer cells. MATERIALS AND METHODS: Cytotoxicity, apoptotic induction and its involved mechanisms of SL4 were investigated using normal cells, cancer cells and mouse xenograft models. The role of reactive oxygen species (ROS) and mitogen-activated protein kinase (MAPK) signalling in SL4-induced apoptosis was explored by manipulating specific scavenger or signalling inhibitors, in cultured cells. RESULTS: SL4 significantly inhibited cell population growth of human cancer cell lines but exhibited lower cytotoxicity against normal cells. In addition, SL4 effectively induced apoptosis of Hep3B and MDA-MB-435 cells by activating procaspase-8, -9 and -3, and down-regulating expression levels of XIAP, but did not affect HIF-1 apoptosis-related targets, Survivin and Bcl-XL. Further study showed that SL4 also reduced mitochondrial membrane potential and promoted generation of ROS. ROS generation and apoptotic induction by SL4 were blocked by NAC, a scavenger of ROS, suggesting SL4-induced apoptosis via ROS accumulation. We also found that MAPKs, JNK and p38, but not ERK1/2, to be critical mediators in SL4-induced apoptosis. SP600125 and SB203580, specific inhibitors of JNK kinase and p38 kinase, significantly retarded apoptosis induced by SL4. Moreover, anti-oxidant NAC blocked activation of JNK and p38 induced by SL4, indicating that ROS may act as upstream signalling of JNK and p38 activation. It is noteworthy that animal studies revealed dramatic reduction (49%) in tumour volume after 11 days SL4 treatment. CONCLUSIONS: These data demonstrate that SL4 induced apoptosis in human cancer cells through activation of the ROS/MAPK signalling pathway, suggesting that it may be a novel lead compound, as a cancer drug candidate, with polypharmacological characteristics.

Effect of RAGE polymorphisms on susceptibility to and severity of osteoarthritis in a Han Chinese population: a case-control study.

Recent studies have revealed that the inflammatory process plays a role in the pathogenesis of osteoarthritis (OA). The S100 family and receptor for advanced glycation end products (RAGE) participate in regulating inflammation, even in the production of matrix metalloproteinases (MMPs). MMP-1 degrades cartilage, which may result in OA development. Moreover, polymorphisms in RAGE, S100A8, and MMP-1 have a marked effect on ligand binding and transcription regulating. In this study, we investigated the potential genetic contribution of the RAGE, S100A8, and MMP-1 genes to OA. We performed a matched case-control association study and genotyped OA patients and healthy controls, who were analyzed by polymerase chain reaction-restriction fragment length polymorphism assays. A total of 207 patients were diagnosed with knee OA and underwent total knee replacement. The control group included 207 individuals who had standard X-rays of the knee joints to confirm K/L < 2 and were matched by age and gender. Single-nucleotide polymorphisms in RAGE (-429T/C, -374T/A, and 557G/A), S100A8 (rs3795391A/G), and MMP-1 (-1607 1G/2G, -755G/T, and -519A/G) were evaluated. RAGE -374T/A, S100A8 rs3795391A/G, MMP-1 -1607 1G/2G, -755G/T, and -519A/G showed no significant difference between OA patients and healthy controls. RAGE -429T/C and 557G/A showed a significant association between OA patients and healthy controls (P = 0.016 and 0.047, respectively). In haplotype analyses, no RAGE and MMP-1 haplotypes showed associations with OA. Our results suggest that the investigated polymorphism in the RAGE gene play a role in OA in the Han Chinese population.

YB-1 disrupts mismatch repair complex formation, interferes with MutSα recruitment on mismatch and inhibits mismatch repair through interacting with PCNA.

Y-box binding protein-1 (YB-1) is highly expressed in tumors and it participates in various cellular processes. Previous studies indicated that YB-1 binds to mispaired DNA and interacts with several mismatch repair (MMR)-related factors. However, its role in the MMR system remains undefined. Here, we found that YB-1 represses mutS homolog 6 (MSH6)-containing MMR complex formation and reduces MutSα mismatch binding activity by disrupting interactions among MMR-related factors. In an effort to elucidate how YB-1 exerts this inhibitory effect, we have identified two functional proliferating cell nuclear antigen (PCNA)-interacting protein (PIP)-boxes that mediate YB-1/PCNA interaction and locate within the C-terminal region of YB-1. This interaction is critical for the regulatory role of YB-1 in repressing MutSα mismatch binding activity, disrupting MutSα/PCNA/G/T heteroduplex ternary complex formation and inhibiting in vitro MMR activity. The differential regulation of 3' and 5' nick-directed MMR activity by YB-1 was also observed. Moreover, YB-1 overexpression is associated with the alteration of microsatellite pattern and the enhancement of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG)-induced and spontaneous mutations. Furthermore, upregulation of other PIP-box-containing proteins, such as myeloid cell leukemia-1 (Mcl-1) and inhibitor of growth protein 1b (ING1b), has no impact on MMR complex formation and mutation accumulation, thus revealing the significant effect of YB-1 on regulating the MMR system. In conclusion, our study suggests that YB-1 functions as a PCNA-interacting factor to exert its regulatory role on the MMR process and involves in the induction of genome instability, which may partially account for the oncogenic potential of YB-1.

Effects of enrofloxacin on cytochromes P4501A and P4503A in Carassius auratus gibelio (crucian carp).

Currently, although enrofloxacin (EF) as a widely used veterinary medicine has begun to apply to treating fish bacterial infections, the researches on the effects of EF on their main drug metabolic enzymes are limited. To investigate the effects of EF on fish cytochromes P450 (CYPs) 1A and 3A, the enzymatic activities and expressions (mRNA and protein) of crucian carp CYP1A and CYP3A after EF administration were examined. For CYP1A, in the in vivo experiments, EF exhibited potent inhibition on the CYP1A-related ethoxyresorufin-O-deethylase (EROD) activity, as well as CYP1A expressions at both protein and mRNA levels, at 24 h after administration with different EF dosages (3, 10, 30, and 60 mg/kg); Furthermore, CYP1A enzymatic activity and expressions at both protein and mRNA levels decreased more with increasing EF dosages. Additionally, the in vitro experimental results showed that, after incubated with microsomes, EF did not change the EROD activity through interacting directly with CYP1A. For CYP3A, the in vitro and in vivo experimental results demonstrated that EF could inhibit the CYP3A-related erythromycin N-demethylase activity in a time- and dose-dependent manner, while it did not suppress CYP3A expressions at both protein and mRNA levels after administration with EF for a short period (no more than 24 h); however, after injection with EF at a high dose (10 mg/kg) for a long period, the CYP3A protein and mRNA reached their lowest levels at 96 and 48 h, respectively. These results indicate that EF can suppress CYP1A expressions in a dose-dependent manner, thereby inhibiting further its catalytic activity; meanwhile, both the interactions of EF with CYP3A and the expressions decrease (protein and mRNA) caused by EF contribute to the CYP3A inhibition.

Effect of temperature on pharmacokinetics of enrofloxacin in mud crab, Scylla serrata (Forsskål), following oral administration.

The study was conducted to evaluate the pharmacokinetics of enrofloxacin following a single oral gavage (10 mg kg(-1)) in mud crab, Scylla serrata, at water temperatures of 19 and 26 degrees C. Enrofloxacin concentration in haemolymph was determined using high-performance liquid chromatography (HPLC). A multiple and repeated haemolymph sampling from the articular cavity of crab periopods was developed. The haemolymph of an individual crab was successfully sampled up to 11 times from the articular cavity. The profile of haemolymph enrofloxacin concentration of an individual crab versus time was thus achieved. The mean haemolymph enrofloxacin concentration versus time was described by a two-compartment model with first-order absorption at two water temperatures. The peak concentrations of haemolymph enrofloxacin at 19 and 26 degrees C were 7.26 and 11.03 mug mL(-1), at 6 and 2 h, respectively. The absorption and distribution half-life time ( and t(1/2alpha)) at 19 degrees C were 3.7 and 4.5 h, respectively, which were markedly larger than the corresponding values (1.1 and 1.5 h) at 26 degrees C; the elimination half-life time (t(1/2beta)) was 79.1 and 56.5 h at 19 and 26 degrees C, respectively. The area under curve (AUC), total body clearance (Cl) and mean residence time (MRT(0-infinity)) at 19 degrees C were 636.0 mg L(-1) h, 0.016 L h(-1) kg(-1) and 102.5 h, respectively; the corresponding values at 26 degrees C were 583.4 mg L(-1) h, 0.018 L h(-1) kg(-1)and 63.7 h. These results indicate that enrofloxacin is absorbed and eliminated more rapidly in mud crab at 26 degrees C than at 19 degrees C.

First results of detection of PRRSV and CSFV RNA by SYBR Green I-based quantitative PCR.

Porcine reproductive and respiratory syndrome (PRRS) and classical swine fever (CSF) cause significant economic losses to the swine industry worldwide. As both diseases cause similar symptoms, rapid and reliable detection of these diseases is essential for disease surveillance. A quantitative SYBR Green I-based reverse transcription-polymerase chain reaction (RT-PCR) is described for simultaneous and differential diagnosis. The established RT-PCR for the quantitation of PRRSV and CSFV cDNA was found to provide a broad dynamic range, detecting from 10(3) to 10(11) and 10(2) to 10(11) copies of cDNA per reaction, respectively. Sensitivity and specificity of this method were compared with those of conventional RT-PCR and both were equal or superior to the reference method. Reproducibility was tested and the assay was proved very reliable. The assay is timesaving, easy to handle, and highly sensitive and specific. Therefore, it is a powerful tool for detecting PRRSV and CSFV simultaneously for routine outbreak investigation.

Inactivation of infectious bursal disease virus by binary ethylenimine and formalin.

In this experiment conducted to study the inactivation dynamics of infectious bursal disease virus (IBDV) by binary ethylenimine (BEI) in comparison with formalin, IBDV was isolated from the bursa of infected chickens and its confirmation was done by agar gel precipitation test. Viral suspensions were subjected to inactivation with BEI and formalin for pre-set time intervals. BEI was employed at concentrations of 0.001 and 0.002 mol/L while formalin was used at 0.1% and 0.2%. Sampling was done at 6, 12, 24, 36 and 48 h of incubation and samples were tested for their inactivation status in 9-day-old embryonated eggs and 3-week-old broiler chickens. IBDV was completely inactivated by 0.001 and 0.002 mol/L BEI after 36 h of incubation at 37 degrees C, whereas formalin at 0.1% and 0.2% concentrations inactivated IBDV in 24 h.

Density functional theory investigation of the reactions of isodihalomethanes (CH(2)X-X where X = Cl, Br, or I) with ethylene: substituent effects on the carbenoid behavior of the CH(2)X-X species.

We investigated the chemical reactions of isodihalomethane (CH(2)X-X) and CH(2)X radical species (where X = Cl, Br, or I) with ethylene and the isomerization reactions of CH(2)X-X using density functional theory calculations. The CH(2)X-X species readily reacts with ethylene to give the cyclopropane product and an X(2) product via a one-step reaction with barrier heights of approximately 2.9 kcal/mol for CH(2)I-I, 6.8 kcal/mol for CH(2)Br-Br, and 8.9 kcal/mol for CH(2)Cl-Cl. The CH(2)X reactions with ethylene proceed via a two-step reaction mechanism to give a cyclopropane product and X atom product with much larger barriers to reaction. This suggests that photocyclopropanation reactions using ultraviolet excitation of dihalomethanes most likely occurs via the isodihalomethane species and not the CH(2)X species. The isomerization reactions of CH(2)X-X had barrier heights of approximately 14.4 kcal/mol for CH(2)I-I, 11.8 kcal/mol for CH(2)Br-Br, and 9.1 kcal/mol for CH(2)Cl-Cl. We compare our results for the CH(2)X-X carbenoids to results from previous calculations of the Simmons-Smith-type carbenoids (XCH(2)ZnX) and Li-type carbenoids (LiCH(2)X) and discuss their differences and similarities as methylene transfer agents.

Effect of recombinant bactericidal/permeability-increasing protein on endotoxin translocation and lipopolysaccharide-binding protein/CD14 expression in rats after thermal injury.

OBJECTIVE: To investigate the potential mechanisms underlying the in vivo effect of recombinant bactericidal/permeability-increasing protein (rBPI21) on endogenous bacteria or endotoxin translocation and lipopolysaccharide-binding protein/CD14 expression secondary to thermal injury. DESIGN: Prospective, randomized, controlled animal study. SETTING: College hospital animal research laboratory. SUBJECTS: Thirty-six male Wistar rats weighing 250-300 g. INTERVENTIONS: The rats were anesthetized, and a 35% total body surface area full-thickness burn was created. Animals were randomized to receive treatment with either rBPI21 or the control protein (albumin). rBPI21 (2 mg/kg body wt, BPI group) or a protein control preparation (burn group) in the same dose was administered in an intravenous bolus at 30 mins and 4 hrs after thermal injury. All animals were killed at 12 and 24 hrs postburn (six to ten rats for each interval). In addition, eight rats were taken as normal controls. MEASUREMENT AND MAIN RESULTS: Our data showed that treatment with rBPI21 was effective in preventing endotoxin translocation secondary to severe burns. Meanwhile, tissue lipopolysaccharide-binding protein, CD14, and tumor necrosis factor-alpha mRNA expression in various organs were inhibited markedly by rBPI21 secondary to acute insults (p <.05-.01). Furthermore, significant reduction in serum aminoleucine transferase concentrations and elevation in intestinal diamine oxidase activities in the rBPI21-treated group were found compared with controls (p <.05-.01). CONCLUSIONS: These findings indicate that endotoxin accumulated in local sites after thermal injury can markedly up-regulate lipopolysaccharide-binding protein/CD14 and tumor necrosis factor-alpha mRNA expression in various organs. Meanwhile, up-regulation of lipopolysaccharide-binding protein/CD14 expression would be the major molecular mechanism of increasing sensitivity to endogenous endotoxin response after burns. Early treatment with rBPI21may be effective in attenuating multiple organ damage resulting from gut-origin endotoxin translocation. This might be associated with the down-regulation effects of tissue lipopolysaccharide-binding protein and CD14 gene expression by the use of rBPI21.

Isodiiodomethane is the methylene transfer agent in cyclopropanation reactions with olefins using ultraviolet photolysis of diiodomethane in solutions: a density functional theory investigation of the reactions of isodiiodomethane, iodomethyl radical, and iodomethyl cation with ethylene.

We examine the chemical reactions of the isodiiodomethane (CH2I-I), .CH2I and CH2I(+) species with ethylene using density functional theory computations. The CH2I-I species readily reacts with ethylene to give the cyclopropane product and an I2 product via a one-step reaction with a barrier height of approximately 2.9 kcal/mol. However, the.CH2I and CH2I(+) species have much more difficult pathways (with larger potential barriers) to react with ethylene via a two-step reaction mechanism. Comparison of experimental results to our present calculation results indicates that the CH2I-I photoproduct species is most likely the methylene transfer agent for the cyclopropanation reaction of olefins via ultraviolet photoexcitation of diiodomethane.

Repair of large insertion/deletion heterologies in human nuclear extracts is directed by a 5' single-strand break and is independent of the mismatch repair system.

The repair of 12-, 27-, 62-, and 216-nucleotide unpaired insertion/deletion heterologies has been demonstrated in nuclear extracts of human cells. When present in covalently closed circular heteroduplexes or heteroduplexes containing a single-strand break 3' to the heterology, such structures are subject to a low level repair reaction that occurs with little strand bias. However, the presence of a single-strand break 5' to the insertion/deletion heterology greatly increases the efficiency of rectification and directs repair to the incised DNA strand. Because nick direction of repair is independent of the strand in which a particular heterology is placed, the observed strand bias is not due to asymmetry imposed on the heteroduplex by the extrahelical DNA segment. Strand-specific repair by this system requires ATP and the four dNTPs and is inhibited by aphidicolin. Repair is independent of the mismatch repair proteins MSH2, MSH6, MLH1, and PMS2 and occurs by a mechanism that is distinct from that of the conventional mismatch repair system. Large heterology repair in nuclear extracts of human cells is also independent of the XPF gene product, and extracts of Chinese hamster ovary cells deficient in the ERCC1 and ERCC4 gene products also support the reaction.

A requirement for protein kinase C inhibition for calcium-triggered apoptosis in acute lymphoblastic leukemia cells.

We have evaluated the cytotoxicities of the combinations of calcium mobilizers and PKC inhibitors against human acute lymphoblastic leukemia (ALL) cells. Here we report that calcium mobilizers alone or PKC inhibitors alone do not induce apoptosis in human ALL cells. However, the combinations of calcium mobilizers with potent inhibitors of PKC cause significant apoptosis in ALL cells. Our results provide experimental evidence that PKC blocks Ca2+-triggered apoptosis in human ALL cells. Thus, PKC inhibitors can be used to enhance the antileukemic activity of chemical or biological agents that trigger an apoptotic calcium signal in ALL cells. The exquisite sensitivity of ALL cells to calcium-dependent apoptosis in the presence of PKC inhibitors could provide the basis for new treatment programs against ALL.

Calphostin C triggers calcium-dependent apoptosis in human acute lymphoblastic leukemia cells.

Recent studies have demonstrated that the naturally occurring perylenequinone antibiotic calphostin C is a potent inhibitor of protein kinase C and can induce apoptosis in some tumor cell lines by an as yet unknown mechanism. Here we demonstrate that calphostin C induces dose-dependent apoptosis in DT40 chicken lymphoma B-cells, and targeted disruption of lyn, syk, btk, PLCgamma2, or IP3R genes does not prevent or attenuate its cytotoxicity. In our study, calphostin C also induced rapid apoptosis in human acute lymphoblastic leukemia (ALL) cell lines ALL-1 (BCR-ABL+ pre-pre-B ALL), RS4;11 (MLL-AF4+ pro-B ALL), NALM-6 (pre-B ALL), DAUDI (Burkitt's/B-cell ALL), MOLT-3 (T-ALL), and JURKAT (T-ALL), whereas other potent PKC inhibitors did not. In biochemical studies, calphostin C was discovered to induce rapid calcium mobilization from intracellular stores of ALL cell lines, and its cytotoxicity against ALL cell lines was well correlated with the magnitude of this calcium signal. Calphostin C-induced apoptosis was markedly suppressed by BAPTA/AM, a cell-permeable Ca2+ chelator as well as NiCl2, an inhibitor of Ca2+/Mg2+-dependent endonucleases. Inhibition of the Ca2+/calmodulin-dependent phosphatase calcineurin with perfluoreperazine dimadeate (a calmodulin antagonist) or cyclosporin A (a specific inhibitor of calcineurin) also reduced the magnitude of calphostin C-induced apoptosis in ALL cell lines. Calphostin C was capable of inducing calcium mobilization and apoptosis in freshly obtained primary leukemic cells from children with ALL. Taken together, our results provide unprecedented evidence that calphostin C triggers a Ca2+-dependent apoptotic signal in human ALL cells.

Altered intermediate filament expression in human neuroblastoma cells transformed by a growth-promoting agent derived from schizophrenic CSF.

1. Transformed (TR) cell lines showed faster doubling times and higher cell densities at confluence, as well as altered morphology, changing from flat epitheloid to smaller round or bipolar shapes. Since such morphological changes are suggestive of alterations in intermediate filaments, we have analyzed the expression of both vimentin and neurofilament. 2. Immunohistochemical analysis of vimentin showed a redistribution from a cytoplasmic network to a perinuclear accumulation in TR cell lines. 3. Western blot analysis demonstrated that the vimentin content was decreased 60-90%. The content of the 70-kD neurofilament protein was also decreased in TR cells, but its intracellular distribution was indistinguishable from that in the control cell lines.

Hypermutability and mismatch repair deficiency in RER+ tumor cells.

A subset of sporadic colorectal tumors and most tumors developing in hereditary nonpolyposis colorectal cancer patients display frequent alterations in microsatellite sequences. Such tumors have been thought to manifest replication errors (RER+), but the basis for the alterations has remained conjectural. We demonstrate that the mutation rate of (CA)n repeats in RER+ tumor cells is at least 100-fold that in RER- tumor cells and show by in vitro assay that increased mutability of RER+ cells is associated with a profound defect in strand-specific mismatch repair. This deficiency was observed with microsatellite heteroduplexes as well as with heteroduplexes containing single base-base mismatches and affected an early step in the repair pathway. Thus, a true mutator phenotype exists in a subset of tumor cells, the responsible defect is likely to cause transitions and transversions in addition to microsatellite alterations, and a biochemical basis for this phenotype has been identified.

An alkylation-tolerant, mutator human cell line is deficient in strand-specific mismatch repair.

The human lymphoblastoid MT1 B-cell line was previously isolated as one of a series of mutant cells able to survive the cytotoxic effects of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). MT1 cells nevertheless remain sensitive to mutagenesis by MNNG and display a mutator phenotype. These phenotypes have been attributed to a single genetic alteration postulated to confer a defect in strand-specific mismatch repair, a proposal that attributes the cytotoxic effect of DNA alkylation in wild-type cells to futile attempts to correct mispairs that arise during replication of alkylated template strands. Our results support this view. MNNG-induced mutations in the HPRT gene of MT1 cells are almost exclusively G.C-->A.T transitions, while spontaneous mutations observed in this mutator cell line are single-nucleotide insertions, transversions, and A.T-->G.C transitions. In vitro assay has demonstrated that the MT1 line is in fact deficient in strand-specific correction of all eight base-base mispairs. This defect, which is manifest at or prior to the excision stage of the reaction, is due to simple deficiency of a required activity because MT1 nuclear extracts can be complemented by a partially purified HeLa fraction to restore in vitro repair. These findings substantiate the idea that strand-specific mismatch repair contributes to alkylation-induced cytotoxicity and imply that this process serves as a barrier to spontaneous transition, transversion, and insertion/deletion mutations in mammalian cells.

Human strand-specific mismatch repair occurs by a bidirectional mechanism similar to that of the bacterial reaction.

Nuclear extracts prepared from a HeLa cell line have been previously shown to support strand-specific repair of heteroduplex DNAs containing a site-specific, strand-specific incision (Holmes, J.J., Clark, S., and Modrich, P. (1990) Proc. Natl. Acad. Sci. U.S.A. 87, 5837-5841; Thomas, D.C., Roberts, J.D., and Kunkel, T.A. (1991) J. Biol. Chem. 266, 3744-3751). Further analysis of the substrate specificity of the reaction has shown that in addition to G-T, A-C, G-G, and C-C, nuclear extracts also recognize and correct in a strand-specific manner A-A, A-G, T-T, and C-T mismatches, with repair in each case being inhibited by aphidicolin. The rate of repair of a circular G-T heteroduplex was found to decrease monotonically with increasing separation between the mismatch and the strand break that targets repair, as viewed along the shorter path joining the two sites in the circular substrate. This decrease is independent of the polarity of the incised strand, suggesting that the human pathway of mismatch correction may possess a bidirectional excision capability similar to that of the Escherichia coli methyl-directed system. This possibility was confirmed by analysis of excision tracts associated with the reaction. Inhibition of DNA synthesis by aphidicolin or by omission of exogenous dNTPs leads to the mismatch-provoked formation of a single-strand gap that spans the shorter path between the strand break and the mismatch, irrespective of the polarity of the incised strand. Formation of these gaps, which extend from the site of the strand break to terminate at a number of discrete sites in the region 90 to 170 nucleotides beyond the mismatch, is therefore independent of the relative orientation of the two sites. Based on similar mismatch specificities and common features of mechanism, we have concluded that the human strand-specific mismatch repair system is functionally homologous to the bacterial methyl-directed pathway.