Knockout of p16 and/or Arf has no effect on TBI-induced reduction of MPPs.p16^−/− (KO), Arf^−/− (KO), p16Arf^−/− (KO), and WT mice were exposed to a sublethal dose (6 Gy) of TBI or were sham irradiated as a control. Two months after TBI, BM cells (BMCs) harvested from the 2 hind legs of individual mice were analyzed by flow cytometry for HPCs, LSK cells, MPPs, and HSCs. The frequencies of HPCs, LSK cells, MPPs, and HSCs in BMCs from each mouse strain are presented as mean ± SD (CTL: n = 4; TBI: n = 6). *P < .05, **P < .01, and ***P < .001, TBI vs control unirradiated mice.