CHEK1 and neoplasm: RB inactive (AdE7 infected) and deficient tumor cells showed elevated p-ATM, p-ATR, p-Chk1, p-Chk2 and p-p53 expressions, and that PSM-RB restored C33A and SaOS2 cells showed weaker expression of those proteins, and that total DNA damage checkpoint proteins were not changed (Figures 1g, h, Supplementary Figures S1c and d).