ATM is a member of the protein superfamily of phosphatidylinositol 3-kinase related serine/threonine kinases (PIKKs). ATM is part of many signaling networks, including cell metabolism and growth, oxidative stress, and chromatin remodeling, all of which can affect cancer progression. Although ATM is considered to be a tumor suppressor, ATM signaling may be advantageous to cancer cells in some settings, particularly in resistance to radio- and chemotherapeutic treatment. Germline ATM mutations are the defining feature of ataxia telangiectasia syndrome, a rare, neurodegenerative, autosomal disorder. ATM somatic mutations are associated with endometrial, colon, pancreatic, breast cancers and urothelial cancers. ATM-mutant cancers are increasingly sensitive to DNA damaging agents due to deficits in DNA repair pathways, and ATM loss may result in better response to checkpoint inhibition in some cancers. For this reason, the use of ATM inhibitors in cancer therapy is under exploration. Genetic alterations of ATM have been identified in 5% of pancreatic adenocarcinomas. ATM F858L has been identified in the scientific literature, but has not been biochemically characterized and therefore, its effect on ATM protein function is unknown. According to ClinVar, this variant is considered to be a benign/likely benign germline variant (https://preview.ncbi.nlm.nih.gov/clinvar/variation/132736/). Clinical correlation is recommended.