Methods for regulating the growth of transplanted cells have many applications in gene and cell therapy. One such method uses conditional signaling molecules that are activated by artificial ligands called chemical inducers of dimerization (CIDs). Here we examine the response of human cord blood cells to a CID-triggered Fibroblast Growth Factor Receptor 1 (F36VFGFR1) signal in vitro and in vivo. In vitro, CD34+ cord blood cells were transduced with a lentivirus vector incorporating a chicken b-actin promoter/cytomegalovirus enhancer (CAG) that also allows for expression in human embryonic stem cells, but which is not expressed in human erythroid cells. Cells cultured in the presence of the CID expanded 40-fold relative to cells cultured in the absence of CID. The cell types evident in culture were predominantly granulocytes/monocytes but B-lymphocytes were also observed. The absence of an effect on erythroid cells is most likely attributable to the lack of transgene expression by the CAG element in erythroid cells. Contrary to our results in murine system using F36VFGFR1, cell expansion was transient, peaking at week 3 of culture. Activation of F36VFGFR1 was associated with a transient increase in CFU-GM. Ex vivo studies using a lentivirus vector in which F36VFGFR1 expression is regulated by an MSCV promoter, allowing for expression in all hematopoietic lineages, have been initiated. Preliminary results from immune deficient mice transplanted with cord blood CD34+ cells transduced with the MSCV based vector lentivirus indicate that F36VFGFR1 activation can support the CID-dependent expansion of a much wider repertoire of hematopoietic lineages, including granulocytes/monocytes, erythroid cells and lymphocytes, compared to our previous studies using a conditional derivative of the thrombopoietin receptor (F36VMpl). These findings establish CID-activated receptors as growth factors for genetically modified human hematopoietic cells.