The c.5763-1050A>G variant in ATM is an intronic variant which results in an intronic A>G substitution before coding exon 38 activating a cryptic splice site leading to alternative splicing. The variant has been observed to cause an insertion of 137 nucleotides of intronic sequence at position 5762 and a premature stop codon at position 1930. Some normal splicing has been reported in patients with ATM c.5763-1050A>G, thus, the splice effect is incomplete (PMIDs 8755918, 10330348, 11382771, 15174027, Ambry internal data). This variant has been detected in many individuals with Ataxia-Telangiectasia, some of whom were described as having a mild presentation and/or later age of onset (PMIDs 8755918, 26896183). The variant has also been reported to segregate with Ataxia-Telangiectasia in 6 affected family members from 3 families (PMIDs 8755918, 15174027). The variant has a minor allele frequency in gnomAD v2.1.1 of 0.00003 (PM2_Supporting, BS1, and BA1 are not met). ATM c.5763-1050A>G has been reported as a founder variant in the British Isles (PMID 9463314). In summary, this variant meets criteria to be classified as pathogenic for autosomal dominant ATM-related cancer predisposition and autosomal recessive Ataxia-Telangiectasia based on the ACMG/AMP criteria applied, as specified by the HBOP VCEP. (PVS1_Strong (RNA), PM3_Very Strong, PP1_Strong)