Balb/c-derived murine chromosome 7 fragment that defines C57BL/6 and HW80 congenic mice. The STAT5ab^–/– C57BL/6 background mouse was crossed onto the HW80 congenic background. Although these mice are very similar, some genetic differences are obvious. C57BL/6 mice have the wild-type tyrosinase gene resulting in black coat color, whereas HW80 congenics have the tyrosinase (c) gene albino mutation that results in white coat color and pink eyes. Another difference between the 2 congenic strains is in the hemoglobin locus. The C57BL/6 mice express an Hb_s variant and the HW80 mice express an Hb_d variant. These differences are of no consequence for transplantation into lethally irradiated recipients and thus can serve as a useful marker for donor engraftment. However, the role of other genes within this chromosome 7 fragment has not been characterized in terms of HSC biology.