ATM/ATR 

ATM/ATR, members of the phosphatidyl inositol 3-kinase-like family of serine/threonine protein kinases (PIKKs), are widely known as being central players in the mitotic DNA damage response (DDR), mounting responses to DNA double-strand breaks (DSBs) and single-stranded DNA (ssDNA) respectively. Activation of ATM by ionizing radiation results in the activation of signal transduction pathways that induce cell cycle arrest at G1/S, S and G2/M. ATR is required for cell cycle arrest in response to DNA-damaging agents such as ultraviolet radiation that cause bulky lesions.

Upon activation, ATM/ATR phosphorylate numerous targets to stabilize stalled replication forks, repair damaged DNA, and inhibit cell cycle progression to ensure survival of the cell and safeguard integrity of the genome. ATM and ATR are central players in activating cell cycle checkpoints and function as an active barrier against genome instability and tumorigenesis in replicating cells.