Loss of Casp9 alters hematopoietic progenitor cell frequency. (A) The frequency of B-cell and erythroid progenitors measured in methylcellulose assays was decreased in Casp9^−/− FLCs compared with Casp9^+/+ FLCs. The frequency and type of myeloid progenitors was not significantly different. (B) The frequency of Kit⁺ lineage^– Sca⁺ (KLS) cells was increased ∼threefold in Casp9^−/− FLCs compared with WT or heterozygous littermates. However, the frequency of long-term stem cells (Kit⁺lineage^–Sca⁺CD48^–CD150⁺; “SLAM”) within the KLS compartment was significantly lower in Casp9^−/− FLCs, resulting in no net alteration in long-term HSC (LT-HSC) abundance. (C) Similar to these in vitro findings, bone marrow harvested from lethally-irradiated WT congenic mice 12 weeks after transplantation with Casp9^−/− FLCs had an increase in KLS and a decrease in SLAM cells within the KLS compartment compared with recipients of Casp9^+/+ FLCs, resulting in a modest decrease in the frequency of LT-HSCs overall. *P < .05, **P < .005, ***P < .0005; ns, not significant.