Immunophenotypic expression profile of multiple myeloma cases at a tertiary hospital in Nairobi Kenya.

Introduction: Multiple myeloma (MM) is a plasma cell neoplasm that constitutes 10-15% of all hematopoietic neoplasms. Kenya is placed among the top five African countries for MM incidence and MM-related mortality. Prior studies have suggested that the aberrant expression of Cyclin D1, CD56, CD117 and Ki-67 on neoplastic plasma cells is useful in disease prognostication. The prevalence and significance of expression of these markers in a cohort of MM cases in Kenya has not been studied previously. Methods: A retrospective cross-sectional study was carried out at the Aga Khan University Hospital, Nairobi. The study population included 83 MM cases with available trephine blocks archived between 1st of January 2009 and 31st of March 2020. Immunohistochemical expression of Cyclin D1, CD56, CD117, and Ki-67 was analyzed and scored. The biomarkers were described using frequencies based on the positive and negative results. Fisher's exact test was used to determine the association between the immunophenotypic markers and categorical variables. Results: Of the 83 selected cases, expression of Cyclin D1, CD56, CD117 and Ki-67 was identified in 28.9, 34.9, 7.2, and 50.6%, respectively. Cyclin D1 positivity was significantly associated with hypercalcemia. Absence of CD117 expression was noted to be associated with adverse risk parameters including an IgA isotype or light chain disease, International Staging System (ISS) stage III disease, abnormal baseline serum free light chains (sFLC) and a high plasma cell burden. Conclusion: Cyclin D1 expression was congruent with previously reported studies. The frequency of CD56 and CD117 expression was lower than previously reported. This may be due to differences in disease biology between the study populations. Approximately half of cases were Ki-67 positive. Our data showed limited associations between the expression of studied markers and clinicopathologic variables. However, this could be attributed to the small study sample size. We would recommend further characterization of the disease in a larger prospective study with the inclusion of survival outcomes and cytogenetic studies.

Diversifying stem cell debates: Including Muslim contexts and perspectives.

Greater transcultural and transdisciplinary engagement within Muslim contexts and deliberate inclusion of diverse Muslim voices in the development of international guidelines is required to improve understanding of the state of stem cell science, strengthen thinking about attendant ethical complexities, enhance compliance, deepen public deliberation, increase trust, and strengthen practice standards.

Oncogene-independent resistance in Philadelphia chromosome - positive (Ph+) acute lymphoblastic leukemia (ALL) is mediated by activation of AKT/mTOR pathway.

Tyrosine kinase inhibitors (TKIs) such as imatinib, nilotinib, dasatinib, and ponatinib have significantly improved the life expectancy of Philadelphia chromosome-positive (Ph+) acute lymphocytic leukemia (ALL) patients; however, resistance to TKIs remains a major clinical challenge. Point mutations in the tyrosine kinase domain (TKD) of BCR-ABL1 have emerged as the predominant cause of acquired resistance. In approximately 30% of patients, the mechanism of resistance to TKIs remains elusive. This study aimed to investigate mechanisms of nonmutational resistance in Ph+ ALL. Here we report the development of a nonmutational resistance cell line SupB15-RT; conferring resistance to approved ABL kinase inhibitors (AKIs) and allosteric inhibitors GNF-2, ABL001, and crizotinib, except for dasatinib (IC90 50nM), a multitarget kinase inhibitor. We found that the AKT/mTOR pathway is activated in these cells and their proliferation inhibited by Torin-1 with an IC50 of 24.7 nM. These observations were confirmed using 3 different ALL patient-derived long term cultures (PDLTCs): (1) HP (BCR-ABL1 negative), (2) PH (BCR-ABL1 positive and responsive to TKIs) and (3) BV (BCR-ABL1 positive and nonmutational resistant to TKIs). Furthermore, Torin-1 and NVP-BEZ235 induced apoptosis in PH and BV cells but not in HP cells. Our experiments provide evidence of the involvement of AKT/mTOR pathway in the evolution of nonmutational resistance in Ph+ ALL which will assist in developing novel targeted therapy for Ph+ ALL patients with BCR-ABL1 independent nonmutational resistance.

Ultra-Structural Imaging Provides 3D Organization of 46 Chromosomes of a Human Lymphocyte Prophase Nucleus.

Three dimensional (3D) ultra-structural imaging is an important tool for unraveling the organizational structure of individual chromosomes at various stages of the cell cycle. Performing hitherto uninvestigated ultra-structural analysis of the human genome at prophase, we used serial block-face scanning electron microscopy (SBFSEM) to understand chromosomal architectural organization within 3D nuclear space. Acquired images allowed us to segment, reconstruct, and extract quantitative 3D structural information about the prophase nucleus and the preserved, intact individual chromosomes within it. Our data demonstrate that each chromosome can be identified with its homolog and classified into respective cytogenetic groups. Thereby, we present the first 3D karyotype built from the compact axial structure seen on the core of all prophase chromosomes. The chromosomes display parallel-aligned sister chromatids with familiar chromosome morphologies with no crossovers. Furthermore, the spatial positions of all 46 chromosomes revealed a pattern showing a gene density-based correlation and a neighborhood map of individual chromosomes based on their relative spatial positioning. A comprehensive picture of 3D chromosomal organization at the nanometer level in a single human lymphocyte cell is presented.

Correction to: Cryo-nanoscale chromosome imaging-future prospects.

A correction to this paper has been published: https://doi.org/10.1007/s12551-020-00767-5 .

Cryo-nanoscale chromosome imaging-future prospects.

The high-order structure of mitotic chromosomes remains to be fully elucidated. How nucleosomes compact at various structural levels into a condensed mitotic chromosome is unclear. Cryogenic preservation and imaging have been applied for over three decades, keeping biological structures close to the native in vivo state. Despite being extensively utilized, this field is still wide open for mitotic chromosome research. In this review, we focus specifically on cryogenic efforts for determining the mitotic nanoscale chromatin structures. We describe vitrification methods, current status, and applications of advanced cryo-microscopy including future tools required for resolving the native architecture of these fascinating structures that hold the instructions to life.

Lack of Androgen Receptor Expression Selects for Basal-Like Phenotype and Is a Predictor of Poor Clinical Outcome in Non-Metastatic Triple Negative Breast Cancer.

Background: Androgen receptor (AR) has emerged as a significant favorable prognostic indicator in estrogen receptor expressing (ER+) breast cancer (BCa); however, its clinical and biological relevance in triple negative breast cancer (TNBC) and association with cancer stem cell (CSC) markers remain ambiguous. Methods: We examined the immunohistochemical expression of AR in a cohort of stage I-III TNBC cases (n = 197) with a long-term clinical follow-up data (mean follow-up = 53.6 months). Significance of AR expression was correlated with prognostic biomarkers including cancer stem cell markers (CD44, CD24, and ALDH1), basal markers (CK5, CK14, and nestin), proliferation marker (ki-67), apoptotic marker (Bcl-2), and COX-2. Expression of CK5 and nestin was used for the categorization of TNBC into basal (TN, CK5+, and/or nestin+) and non-basal (TN, CK5-, and/or nestin-) phenotypes, and Kaplan-Meier curves were used for estimation of overall survival and breast cancer-specific survival (BCSS). Results: AR expression was observed in 18.8% of non-metastatic TNBC tumors. Expression of AR correlated with lower grade (P < 0.001) and conferred a favorable prognostic significance in patients with axillary lymph node metastasis (P = 0.005). Lack of AR expression correlated with expression of CSC phenotype (CD44+/CD24-) (P < 0.001), COX-2 (P = 0.02), basal markers (CK5: P = 0.03), and nestin (P = 0.01). Basal-like phenotype (TN, CK5+, and/or nestin+) correlated with quadruple-negative breast cancer (QNBC) and showed a significant association with adverse prognostic markers including high proliferation index (P < 0.001), expression of COX-2 (P = 0.009), and CSC phenotype (CD44+/CD24-: P = 0.01). Expression of AR remained an independent prognostic indicator for improved overall survival (P = 0.003), whereas basal-like phenotype was associated with an adverse BCSS (P = 0.013). Conclusions: Assessment of AR and basal markers identified biologically and clinically distinct subgroups of TNBC. Expression of AR defined a low-risk TNBC subgroup associated with improved overall survival, whereas expression of basal markers (CK5 and nestin) identified a high-risk subgroup associated with adverse BCSS. Integration of immunohistochemical analysis of AR and basal biomarkers to the assessment of TNBC tumors is expected to improve the prognostication of an otherwise heterogeneous disease.

First meeting in Asia of the Asia Pacific Research Integrity network.

In 2017, the University of Hong Kong and the University of California San Diego co-hosted the first Asian meeting of the recently formed Asia Pacific Research Integrity (APRI) network in Hong Kong. Aligned with planning meetings in 2015 and 2016 funded in part by the US Office of Research Integrity (ORI), the Hong Kong meeting was designed by a multi-national planning committee to address pressing challenges in research integrity: improving multi-national communication; exchanging information on managing misconduct investigations; and sharing best practices to promote research integrity. To create a sustainable, robust international partnership to promote research integrity in the region, the purpose of this 2017 meeting was to foster multi-national awareness, understanding, and opportunities for collaboration. The meeting was defined by four objectives that emerged from the previous meetings: (1) Articulate differences as well as areas of common ground; (2) Identify best or recommended practices; (3) Identify opportunities for research or collaboration; and (4) Set an APRI network agenda for coming years. The key anticipated outcome was to advance the conversation surrounding research integrity among academic institutions and regulators in Asian and Pacific Rim nations. This outcome was evidenced by meeting participation, participant satisfaction, and articulation of next steps for the APRI network.

LMP1 expression in bone marrow trephines of patients with multiple myeloma confers a survival advantage.

Multiple myeloma (MM) is a heterogeneous disease of the bone marrow (BM). Its association with Epstein-Barr virus (EBV) remains enigmatic. Aim of our study was to determine expression of latent membrane protein 1 (LMP1), aldehyde dehydrogenase 1 (ALDH1), CD117 and their association with 5-year survival in MM patients. Seven percent of cases expressed LMP1 in MM cells with no association with survival. Whereas, LMP1 expression in CD138- non-neoplastic cells was observed in 80% of the cases, conferring a survival advantage of 1.75 years (mean 3.75 ± 0.28, 95% CI 3.19-4.3). LMP1 in CD138- non-neoplastic cells was associated with CD117 expression in MM cells. Combinatorial analysis of LMP1 and CD117 stratified patients into good prognostic group LMP1+/CD117- (mean survival 4.16 ± 0.39 years) and a worst prognostic group; LMP1-/CD117+ (mean survival 1.02 ± 0.29 years). Our study showed that LMP1 expression in CD138- non-neoplastic cells of BM in MM patients confers a survival advantage.

Expression of Androgen Receptor and Cancer Stem Cell Markers (CD44+/CD24- and ALDH1+): Prognostic Implications in Invasive Breast Cancer.

BACKGROUND: Androgen receptor (AR) has emerged as a significant prognostic marker in early breast cancer (BCa). Association of AR with cancer stem cell (CSC) markers in BCa is unknown. Aim of the present study was to evaluate the immunohistochemical expression of AR, CD44, CD24 and ALDH1 in a cohort of Pakistani patients diagnosed with invasive BCa and to correlate the expression with 5- year disease free survival. PATIENTS AND METHODS: We evaluated immunohistochemical expression AR, CD44, CD24 and ALDH1 in formalin fixed paraffin embedded archival blocks of 166 cases of primary invasive BCa (stage I-III) and correlated the expression with clinicopathological variables and outcome using univariable and multivariable analysis. Survival data was computed by Kaplan Meier curves. RESULTS: Expression of AR was observed in 62.7% tumors whereas CD44, CD24 and ALDH1 were expressed in 61.4%, 44% and 30.1% tumors, respectively. AR expression was significantly associated with T1-T2 tumors, lower grade, estrogen and progesterone receptor expression (P < .05) and remained an independent prognostic indicator in multivariable analysis (adjusted HR 0.33, 95% CI 0.13-0.81; P = .016). Significant association was observed between concordant expression of AR and CD24 (P = .001) with a favorable impact on survival (P = .007) whereas expression of CSC phenotypes (CD44+, CD44+/CD24- and ALDH1+) did not correlate with adverse outcome (P > .05). However, AR expression retained the association with better prognosis even in patients whose tumors exhibited a CSC phenotype. CONCLUSIONS: Expression of AR and CD24 in stage I-III invasive BCa correlates with favorable clinicopathological features and delineates a subgroup of patients with better disease-free survival.

Sociodemographic factors associated with IgG and IgM seroprevalence for human cytomegalovirus infection in adult populations of Pakistan: a seroprevalence survey.

BACKGROUND: The seroprevalence of human cytomegalovirus (HCMV) infection ranges from 30 to 90 % in developed countries. Reliable estimates of HCMV seroprevalence are not available for Pakistan. This study determined the seroprevalence and sociodemographic factors associated with HCMV infection in adult populations of Karachi, Pakistan. METHODS: A seroprevalence survey was conducted on 1000 adults, including residents of two semi-urban communities, and visitors to a government and a private hospital. Questionnaire-based interviews were conducted. Sera were analysed for HCMV-specific IgG and IgM. Chi-square or Fisher's exact test was used for comparing sociodemographic variables against seropositivity of HCMV-IgG or IgM. Multiple logistic regression modeling was performed for IgG seroprevalence and adjusted odds ratios were computed. RESULTS: The seroprevalence of HCMV-IgG and IgM was 93.2 and 4.3 % respectively. 95.3 % of individuals who were IgM seropositive were also seropositive for IgG. Around 6 % (15/250) of women of childbearing age remained uninfected and were therefore susceptible to primary infection. HCMV-IgG seroprevalence was associated with being female (p = 0.001), increasing age (p = 0.002) and crowding index (p = 0.003) and also with lower levels of both education (p < 0.001) and income (p = 0.008). Seroprevalence also differed significantly by marital status (p = 0.008) and sampling location (p < 0.001). A logistic regression model for HCMV-IgG seroprevalence showed associations with being female (OR = 1.89; 95 % CI: 1.10-3.25), increasing age (OR = 3.95; 95 % CI: 1.79-8.71) and decreasing income (OR = 0.72; 95 % CI: 0.54-0.96). A strong association was observed between increased seroprevalence of HCMV-IgM and decreasing household size (p = 0.008). CONCLUSIONS: Seroprevalence of HCMV is very high in Pakistan, although 6 % of women of childbearing age remain at risk of primary infection. The IgM seropositivity observed in some individuals living in small household size (1-3 individuals) with persistent HCMV infection could have resulted from a recurrent HCMV infection. Future longitudinal research in pregnant women and neonates is required to study the trends in HCMV seroprevalence over time in Pakistan for the development of a potential HCMV prevention and vaccination programme.

Expression of the Androgen Receptor, pAkt, and pPTEN in Breast Cancer and Their Potential in Prognostication.

BACKGROUND: Importance of androgen receptor (AR) as an independent prognostic marker in Pakistani women with breast cancer (BCa) remains unexplored. Our aim was to identify the expression and potential prognostic value of AR, its upstream regulator (pAkt) and target gene (pPTEN) in invasive BCa. METHODS: This study used a cohort of 200 Pakistani women with invasive BCa diagnosed during 2002-2011. Expression of AR, pAkt and pPTEN was determined on formalin fixed paraffin embedded tissue sections by immunohistochemistry. The association of AR, pAkt and pPTEN with clinicopathological parameters was determined. Survival analyses were undertaken on patients with ≥5years of follow-up (n=82). RESULTS: Expression of AR, pAkt and pPTEN was observed in 47.5%, 81.3% and 50.6% of patients, respectively. AR-expressing tumors were low or intermediate in grade (P<.001) and expressed ER (P=.002) and PR (P=.001). Patients with AR+ tumors had significantly higher OS (Mean OS=10.2±0.465years) compared to patients with AR- tumors (Mean OS=5.8±0.348years) (P=.047). Furthermore, AR-positivity was associated with improved OS in patients receiving endocrine therapy (P=.020). Patients with AR+ /pAkt+ /pPTEN- tumors, had increased OS (Mean OS=7.1±0.535years) compared to patients with AR-/pAkt+/pPTEN- tumors (Mean OS=5.1±0.738years). CONCLUSION: AR-expressing tumors are frequently characterized by low or intermediate grade tumors, expressing ER and PR. In addition, expression of AR, pAkt and pPTEN, could be considered in prognostication of patients with invasive BCa.

Nuclear magnetic resonance detects phosphoinositide 3-kinase/Akt-independent traits common to pluripotent murine embryonic stem cells and their malignant counterparts.

Pluripotent embryonic stem (ES) cells, a potential source of somatic precursors for cell therapies, cause tumors after transplantation. Studies of mammalian carcinogenesis using nuclear magnetic resonance (NMR) spectroscopy have revealed changes in the choline region, particularly increased phosphocholine (PCho) content. High PCho levels in murine ES (mES) cells have recently been attributed to cell pluripotency. The phosphoinositide 3-kinase (PI3K)/Akt pathway has been implicated in tumor-like properties of mES cells. This study aimed to examine a potential link between the metabolic profile associated with choline metabolism of pluripotent mES cells and PI3K/Akt signaling. We used mES (ES-D3) and murine embryonal carcinoma cells (EC-F9) and compared the metabolic profiles of 1) pluripotent mES (ESD0), 2) differentiated mES (ESD14), and 3) pluripotent F9 cells. Involvement of the PI3K/Akt pathway was assessed using LY294002, a selective PI3K inhibitor. Metabolic profiles were characterized in the extracted polar fraction by (1)H NMR spectroscopy. Similarities were found between the levels of choline phospholipid metabolites (PCho/total choline and PCho/glycerophosphocholine [GPCho]) in ESD0 and F9 cell spectra and a greater-than five-fold decrease of the PCho/GPCho ratio associated with mES cell differentiation. LY294002 caused no significant change in relative PCho levels but led to a greater-than two-fold increase in PCho/GPCho ratios. These results suggest that the PCho/GPCho ratio is a metabolic trait shared by pluripotent and malignant cells and that PI3K does not underlie its development. It is likely that the signature identified here in a mouse model may be relevant for safe therapeutic applications of human ES cells.

CD151 regulates tumorigenesis by modulating the communication between tumor cells and endothelium.

The tetraspanin CD151 forms stoichiometric complexes with laminin-binding integrins (e.g., alpha3beta1, alpha6beta1, and alpha6beta4) and regulates their ligand-binding and signaling functions. We have found that high expression of CD151 in breast cancers is associated with decreased overall survival (3.44-fold higher risk of death). Five-year estimated survival rates were 45.8% (95% confidence interval, 16.4-71.4%) for CD151-positive patients and 79.9% (95% confidence interval, 62.2-90.0%) for CD151-negative patients. Furthermore, CD151 was positively associated with axillary lymph node involvement. To study the biological significance of this observation, we investigated the contribution of CD151 in breast cancer tumorigenesis using MDA-MB-231 cells as a model system. Stable down-regulation of this tetraspanin by short-hairpin RNA decreased the tumorigenicity of these cells in mice. Detailed immunohistologic analysis of CD151(+) and CD151(-) xenografts showed differences in tumor vascular pattern. Vascularization observed at the subcutaneous border of the CD151(+) tumors was less pronounced or absent in the CD151(-) xenografts. In vitro experiments have established that depletion of CD151 did not affect the inherent proliferative capacity of breast cancer cells in three-dimensional extracellular matrices, but modified their responses to endothelial cells in coculture experiments. The modulatory activity of CD151 was dependent on its association with both alpha3beta1 and alpha6beta4 integrins. These data point to a new role of CD151 in tumorigenesis, whereby it functions as an important regulator of communication between tumor cells and endothelial cells. These results also identify CD151 as a potentially novel prognostic marker and target for therapy in breast cancer.

Predictive value of PTEN and AR coexpression of sustained responsiveness to hormonal therapy in prostate cancer--a pilot study.

One limitation of current biochemical or histologic analysis of advanced prostate cancer (PC; T(3)/T(4) +/- N(x) M(x)) is the ability to identify on first diagnostic biopsy patients who will make a durable response to hormone ablation therapy. The aim of this study was to assess the predictive value (sustained response to hormonal therapy and clinical outcome (relapse-free and overall survival)) of phosphatase and tensin homolog (PTEN) and the androgen receptor (AR) immunoexpression in the presenting biopsy. Analysis was performed on 47 samples (10 cases of benign prostatic hyperplasia and 37 hormone-naive PCs). Patients selected represented two stages in the natural history of PC: The "clinical metastatic androgen-responsive" (androgen-dependent PC, ADPC) and the "clinical metastatic androgen-resistant" (androgen-independent PC, AIPC). Reduced immunoreactivity (IR) of either or both PTEN/AR in the initial hormone-naive PC samples was observed with increased frequency in AIPCs. In the ADPC group, low PTEN and/or AR-IR was associated with a shorter median relapse-free survival, i.e., at 30 months after surgery, the probability of relapse-free survival for high expressors of PTEN and AR was 85.7% (SEM = 9.3) compared with only 16.6% (SEM = 15.2) in low expressors. At 36 months, only 28.5% (SEM = 9.3) of ADPC high expressors had experienced a biochemical relapse compared with 100% of low expressors (hazard ratio, 4.6; 95% confidence interval, 4.7-146.8). Further studies analyzing the coexpression of PTEN and AR should be undertaken to validate this pilot study and the utility of these biomarkers in routine histopathologic workup of patients with PC.

Early hormonal data from a multicentre phase II trial using transdermal oestrogen patches as first-line hormonal therapy in patients with locally advanced or metastatic prostate cancer.

OBJECTIVE: To assess the hormonal effects of Fem7 (Merck, KGaA, Darmstadt, Germany) 100 microg transdermal oestrogen patches on men undergoing first-line androgen-deprivation therapy for prostate cancer. PATIENTS AND METHODS: PATCH is a multicentre, randomized, phase II trial for men with locally advanced or metastatic prostate cancer, comparing luteinizing hormone-releasing hormone agonist therapy with oestrogen patches. To assess the dosing schedule for the patches, as this was the first time that this brand of patch had been used in men, and to reassure patients and participating clinicians, the Independent Data Monitoring Committee agreed to early release of hormonal data from this study. RESULTS: Oestradiol, testosterone and prostate-specific antigen (PSA) levels are presented for the first group of 14 patients who received the patches (with 1 withdrawal) and for whom there were > or =12 weeks of follow-up by March 2007. After 12 weeks, testosterone levels (nmol/L) in eight of the 13 patients were <1.7, two were 1.7-2 and three were >2. The median (range) serum oestradiol levels was 442 (52.1-1542) pmol/L and all patients had a PSA response, with eight having a PSA level of <4 ng/mL. CONCLUSION: These results confirm that oestrogen patches produce castrate levels of testosterone and concomitant PSA responses. They also highlighted the potential differences between different brands of oestrogen patches, and the need to monitor hormonal response, toxicity and efficacy until more experience with oestrogen patches for this clinical indication is obtained. The number of patches recommended in the PATCH study has now been increased.

Therapy Insight: parenteral estrogen treatment for prostate cancer--a new dawn for an old therapy.

Oral estrogens were the treatment of choice for carcinoma of the prostate for over four decades, but were abandoned because of an excess of cardiovascular and thromboembolic toxicity. It is now recognized that most of this toxicity is related to the first pass portal circulation, which upregulates the hepatic metabolism of hormones, lipids and coagulation proteins. Most of this toxicity can be avoided by parenteral (intramuscular or transdermal) estrogen administration, which avoids hepatic enzyme induction. It also seems that a short-term but modest increase in cardiovascular morbidity (but not mortality) is compensated for by a long-term cardioprotective benefit, which accrues progressively as vascular remodeling develops over time. Parenteral estrogen therapy has the advantage of giving protection against the effects of andropause (similar to the female menopause), which are induced by conventional androgen suppression and include osteoporotic fracture, hot flashes, asthenia and cognitive dysfunction. In addition, parenteral estrogen therapy is significantly cheaper than contemporary endocrine therapy, with substantive economic implications for health providers.

p27Kip1 and p130 cooperate to regulate hematopoietic cell proliferation in vivo.

To investigate the potential functional cooperation between p27Kip1 and p130 in vivo, we generated mice deficient for both p27Kip1 and p130. In p27Kip1-/-; p130-/- mice, the cellularity of the spleens but not the thymi is significantly increased compared with that of their p27Kip1-/- counterparts, affecting the lymphoid, erythroid, and myeloid compartments. In vivo cell proliferation is significantly augmented in the B and T cells, monocytes, macrophages, and erythroid progenitors in the spleens of p27Kip1-/-; p130-/- animals. Immunoprecipitation and immunodepletion studies indicate that p130 can compensate for the absence of p27Kip1 in binding to and repressing CDK2 and is the predominant CDK-inhibitor associated with the inactive CDK2 in the p27Kip1-/- splenocytes. The finding that the p27Kip1-/-; p130-/- splenic B cells are hypersensitive to mitogenic stimulations in vitro lends support to the concept that the hyperproliferation of splenocytes is not a result of the influence of their microenvironment. In summary, our findings provide genetic and molecular evidence to show that p130 is a bona fide cyclin-dependent kinase inhibitor and cooperates with p27Kip1 to regulate hematopoietic cell proliferation in vivo.