BMI1 mediates DNA damage–induced γ-H2AX monoubiquitylation. U2OS cells transfected with either control or a BMI1 shRNA were irradiated (6 Gy) and permitted to recover for 1 h at 37°C. (A and B) Histone extracts were prepared and immunoblotted with γ-H2AX (A) and H2AX antibodies (B). Black lines indicate that intervening lanes have been spliced out. RNF8 WT and RNF8 KO were treated as in A. (C and D) Histone extracts were prepared and immunoblotted with γ-H2AX (C) and H2AX (D) antibodies. (E and F) RNF8 KO cells transfected with either control shRNA or one of two different BMI1 shRNA for 24 h and were treated as in A and B.