Ectopic expression of Jak3 in normal mouse BM cells results in accelerated macrophage differentiation in response to GM-CSF. Following infection of myeloblast-enriched BM cells from wild-type (C57BL/6) with pMSCV-neo or pMSCV-Jak3 retroviral vectors, cells were induced for differentiation with GM-CSF. Morphology was assessed by May-Grunwald and Giemsa staining at the indicated time points. Original magnification, × 20.