53BP 1 has been reported to have multiple roles in mammalian DNA damage response (DDR), specifically for repair of DNA double-strand breaks (DSBs) in an ATM-dependent manner that involves H2AX phosphorylation (γ H2AX) and recruitment of MDC1, RNF8,and RNF168.53BP1 is critical for nonhomologous end-joining (NHEJ)-mediated DNA DSB repair and has been implicated in the activation of intra-S-phase and G2/M checkpoints.