ATM and ataxia telangiectasia: Biallelic ATM loss-of-function (LOF) variants result in ataxia–telangiectasia (AT), also known as Louis–Barr syndrome, an autosomal recessive disorder characterized by progressive cerebellar degeneration, ocular telangiectasias, immunodeficiency, and radiosensitivity, as well as predisposition to several hematologic and solid cancers.