Serine/threonine protein kinase which activates checkpoint signaling upon genotoxic stresses such as ionizing radiation (IR), ultraviolet light (UV), or DNA replication stalling, thereby acting as a DNA damage sensor. Recognizes the substrate consensus sequence [ST]-Q. Phosphorylates BRCA1, CHEK1, MCM2, RAD17, RPA2, SMC1 and p53/TP53, which collectively inhibit DNA replication and mitosis and promote DNA repair, recombination and apoptosis. Phosphorylates 'Ser-139' of histone variant H2AX at sites of DNA damage, thereby regulating DNA damage response mechanism. Required for FANCD2 ubiquitination. Critical for maintenance of fragile site stability and efficient regulation of centrosome duplication. Positively regulates the restart of stalled replication forks following activation by the KHDC3L-OOEP scaffold complex (By similarity). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Serine/threonine-protein kinase activity is directly stimulated by TOPBP1 (PubMed:16530042). ATR kinase activity is also directly activated by ETAA1, independently of TOPBP1 (PubMed:27723720, PubMed:27723717). Activated by DNA and inhibited by BCR-ABL oncogene (PubMed:10597277). Slightly activated by ATRIP (PubMed:14729973). Inhibited by caffeine, wortmannin and LY294002 (PubMed:9766667). Forms a heterodimer with ATRIP.(PubMed:11721054). Binds to DNA, and to UV-damaged DNA with higher affinity. Interacts with RAD17, MSH2 and HDAC2. Present in a complex containing ATRIP and RPA-coated single-stranded DNA. Present in a complex containing CHD4 and HDAC2. Interacts with EEF1E1, the interaction is enhanced by UV irradiation. Interacts with CLSPN and CEP164. Interacts with TELO2 and TTI1. Interacts with BCR-ABL after genotoxic stress. Interacts with UHRF2; this interaction promotes ATR activation. Depending on the cell type, it can also be found in PML nuclear bodies. Recruited to chromatin during S-phase. Redistributes to discrete nuclear foci upon DNA damage, hypoxia or replication fork stalling. Ubiquitous, with highest expression in testis. Isoform 2 is found in pancreas, placenta and liver but not in heart, testis and ovary. Phosphorylated; autophosphorylates in vitro. The disease is caused by variants affecting the gene represented in this entry. The disease is caused by variants affecting the gene represented in this entry. Belongs to the PI3/PI4-kinase family. ATM subfamily.