Hematopoietic stem cell gene therapy has not provided clinical success in disorders such as chronic granulomatous disease (CGD), where genetically corrected cells do not show a selective advantage in vivo. To facilitate selective expansion of transduced cells, we have developed a fusion receptor system that confers drug-induced proliferation. Here, a 'selective amplifier gene (SAG)' encodes a chimeric receptor (GcRER) that generates a mitotic signal in response to estrogen. We evaluated the in vivo efficacy of SAG-mediated cell expansion in a mouse disease model of X-linked CGD (X-CGD) that is deficient in the NADPH oxidase gp91(phox) subunit. Bone marrow cells from X-CGD mice were transduced with a bicistronic retrovirus encoding GcRER and gp91(phox), and transplanted to lethally irradiated CGD recipients. Estrogen was administered to a cohort of the transplants, and neutrophil superoxide production was monitored. A significant increase in oxidase-positive cells was observed in the estrogen-treated mice, and repeated estrogen administration maintained the elevation of transduced cells for 20 weeks. In addition, oxidase-positive neutrophils were increased in the X-CGD transplants given the first estrogen even at 9 months post-transplantation. These results showed that the SAG system would enhance the therapeutic effects by boosting genetically modified, functionally corrected cells in vivo.