ATR (ATM and Rad3 related) is closely related to ATM (Ataxia telangiectasia, mutated) and is a member of the phosphatidylinositol 3 kinase (PI-3) family that is an early sensor of DNA damage. ATR is a serine-threonine kinase that reacts to UV damage and interruptions in replication. ATR may be able to sense DNA damage through interaction with Rad17 and 1as well as components of nucleosome remodeling complexes. In response to DNA damage, ATR has been shown to phosphorylate a multitude of substrates which include BRCA1, p53, Chk2, Rad 17, and E2F transcription factor 1.