Engraftment of HW80 congenic STAT5ab^–/– BM cells following injection into nonirradiated W/W^v hosts. To determine whether the congenic background would alter the HSC repopulating ability in W/W^v hosts, BM cells were isolated from HW80 STAT5ab^–/– congenic mice (n = 4) or wild-type HW80 mice (n = 4) and injected in 3 separate transplants into W/W^v hosts. As a control, wild-type HW80 bone marrow cells were injected into W/W^v recipients. Engraftment levels were determined by hemoglobin electrophoresis 17 weeks following injection. At the right are Hb_s, Hb_d, and W/W^v controls. At the time of death, BM and spleen tissue was collected from transplanted mice injected on 2 separate occasions with either STAT5ab^–/– (n = 3) or wild-type (n = 4) BM cells. Donor number below each set indicates which of the primary recipients were from the same donors. Mice were analyzed for the donor engraftment by Southern blot using a hemoglobin probe. The percentage of Hb_d is also shown below each lane to indicate the level of donor HW80 contribution. Hb_s, Hb_d, and W/W^v controls are shown on this blot.