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The ability to expand hematopoietic stem and progenitor cells (HSPCs) in vitro will enhance the success of a wide range of transplant-related therapies. PTEN (phosphatase and tensin homologue deleted on chromosome 10) has been implicated as a regulator of murine HSPC self-renewal, but little is understood about the role of PTEN in human HSPC regulation. We tested the impact of transient small interfering RNA (siRNA)–induced inhibition of PTEN expression in human CD34+ cells on their cell cycle profile, their susceptibility to retroviral transduction, and their ability to self-renew and repopulate nonobese diabetic/severe combined immunodeficiency disease with interleukin-2 receptor γ-chain deficiency mice. Reduced PTEN messenger RNA and protein levels were confirmed in PTEN siRNA-treated CD34+ cells compared with control siRNA-treated CD34+ cells. Transient silencing of PTEN in CD34+ cells promoted their entry into cell cycle, and increased their expansion in vitro compared with control siRNA-treated CD34+ cells. When these cells were transduced with retroviral vectors, transduction efficiencies in the bulk CD34+ cells transfected with PTEN siRNA were significantly higher compared with CD34+ cells transfected with a control siRNA. Transient PTEN suppression in CD34+ cells also increased their proliferation and engraftment potential in nonobese diabetic/severe combined immunodeficiency disease with interleukin-2 receptor γ-chain deficiency mice, and maintained their multilineage differentiation capacity in vivo. No mice developed myeloproliferative disorders or leukemias. Similar to findings with murine HSPC, PTEN may also promote quiescence of human HSPC. With optimization of technologies for transfer of siRNA in primary CD34+ cells, this approach may facilitate investigations into the mechanisms underlying HSPC self-renewal, and could find clinical applications in gene therapy protocols. [Copyright &y& Elsevier]