ATM/ATR are members of the PI3 family of serine-threonine kinases and function as essential links between the sensors and effectors of the DNA damage response. The roles of ATM and ATR partially overlap and are cooperative; however they are also known to play distinct roles in protecting the cell from DNA damage. ATM is mostly responsible for sending signals from DSBs (double-strand breaks) induced by ionizing radiation while the closely related ATR responds to UV damage or stalled replication forks. ATM and ATR are known to phosphorylate common as well as specific substrates to activate checkpoint signaling. The G1, S, and G2 cell cycle checkpoints are primarily regulated by the ATM (ataxia telangiectasia, mutated) and ATR (ATM and Rad3-related) protein kinases.