The phosphatidylinositol kinase (PIK) family members fall into two distinct subgroups. The first subgroup contains proteins such as the PI 3- and PI 4- kinases and the second group comprises the PIK-related kinases. The PIKrelated kinases include Atm, DNA-PKCS and FRAP. These proteins have in common a region of homology at their carboxy-termini that is not present in the PI 3- and PI 4-kinases. The Atm gene is mutated in the autosomal recessive disorder ataxia telangiectasia (AT) that is characterized by cerebellar degeneration (ataxia) and the appearance of dilated blood vessels (telangiectases) in the conjunctivae of the eyes. AT cells are hypersensitive to ionizing radiation, impaired in mediating the inhibition of DNA synthesis and they display delays in p53 induction. Cultured cells respond to breaks in double stranded DNA by immediate activation of ATM through autophosphorylation on Serine 1981. Most human tissues, however, contain the nonphosphorylated, inactive form of Atm.Synonyms: A-T mutated, ATDC, Ataxia telangiectasia mutated, Serine-protein kinase ATM, TEL1, TELO1