ATM and cancer: Our results are consistent with the literature reports that inhibition of ATM and DNA-PKcs phosphorylation by KU55933 can result in the suppression of H2AX phosphorylation and etoposide-induced apoptosis in resting HPBLs.26,58 It is believed that HPBLs in a resting status tolerate a limited un-repair and inaccurate repair, and that there are opposite actions of KU55933 on normal resting cells and proliferating cancer cells.