ATR (ataxia telangiectasia and Rad-3-related) is one of the most important checkpoint proteins in mammalian cells responding to DNA damage. ATR is a member of the phosphatidylinositol kinase-related protein family that includes ATM (ataxia telangiectasia mutated). These kinases are essential for signaling the presence of DNA damage or replication blocks and activating cell cycle checkpoints. It is reported that checkpoint kinase 1 (CHK1), RAD9, RAD17, Histone H2AX, p53, and BRCA1 are phosphorylated in response to genotoxic stress in an ATR-dependent manner. ATR also phosphorylates ATR-interacting protein (ATRIP) that regulates ATR expression, and is an essential component of the DNA damage checkpoint signaling pathways. In addition, homozygous deletion of ATR in mice causes early embryonic lethality, suggesting that ATR has essential functions during development.Synonyms: Ataxia telangiectasia and Rad3-related protein, FRAP-related protein 1, Serine/threonine-protein kinase ATR