Female NSG mice more efficiently support the self-renewal of human HSCs. (A) Whole bone marrow (IF and BM) was harvested and transplanted in parallel into secondary male and female recipients at equivalent cell dose. Mice were killed 12 weeks after transplantation, and human cell engraftment was assessed by flow cytometry. Shown is the representative analysis from 2 of 4 independent cases. (B) Whole bone marrow from primary male mice displaying low (> 0.1%, n = 4) or undetectable (< 0.1%, n = 8) levels of human engraftment were transplanted into secondary female recipients to evaluate whether males harbored quiescent HSCs. No engraftment was detected in secondary females. Shown is representative analysis from 3 cases.