Loss of CFU potential after exposure to IR is dependent on Ink4a/arf status. BM-SCs (A) and OB-SCs (B) were isolated from wild-type (WT) or Ink4a/arf-deficient (KO) mice exposed (IR) or not (CTR) to 6 Gy of IR 8 weeks before being killed and cells were fixed after 2 weeks of culture to determine CFUs (average ± SEM). n = 9-19 mice per group. P values from Student t tests are shown.