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Human embryonic stem cells (hES Cs) are an attractive alternative cell source for hematopoietic gene therapy applications as the cells are easily modified with lentiviral or other vectors and can be subsequently induced to differentiate into hematopoietic progenitor cells. However, demonstration of the full hematopoietic potential of hESC-derived progeny is challenging due to low marrow engraftment and the difficulty of detecting cells in the peripheral blood of human/mouse xenografts. Methotrexate (MTX) chemotherapy coupled with expression of a drug resistant dihydrofolate reductase such as Tyr22 (Tyr22DHFR) has the potential to selectively increase engraftment of gene-modified human hematopoietic cells in mice, which would allow for better phenotypic characterization of hESC-derived cells in vivo. We showed that hES Cs transduced with Tyr22DHFR-GFP encoding lentivirus vectors differentiate into MTX resistant (MTXr) hemato-endothelial cells. MTX treatment of immunodeficient mice infused with Tyr22DHFR hESC-derived hemato-endothelial cells increased the long-term engraftment of human cells in the bone marrow of MTX-treated mice. In contrast to previous studies, these results indicate that MTX administration has the potential to support in vivo selection that is maintained after cessation of treatment. The MTX/Tyr22DHFR system may therefore be useful for enrichment of gene-modified cell populations in human stem cell and gene therapy applications., (© 2010 Landes Bioscience)