The c.824del (p.Leu275Ter) variant in ATM is a frameshift variant predicted to cause a premature stop codon in a biologically-relevant-exon leading to nonsense mediated decay in a gene in which loss-of-function is an established disease mechanism. This alteration results in a termination codon upstream of the most C-terminal pathogenic alteration (ATM p.Arg3047*), as classified by the HBOP VCEP, and is expected to be more deleterious. This variant has been detected in at least 5 individuals with Ataxia-Telangiectasia (PMID: 26896183, 21792198, 9463314, 9887333). The c.824del (p.Leu275Ter) variant in ATM has a minor allele frequency in gnomAD v2.1.1 of 0.000018 in the non-Finnish European population (PM2_Supporting, BS1, and BA1 are not met). In summary, this variant meets criteria to be classified as pathogenic for autosomal dominant hereditary breast cancer and autosomal recessive Ataxia-Telangiectasia based on the ACMG/AMP criteria applied, as specified by the HBOP VCEP. (PVS1, PM5_Supporting, PM3_VeryStrong)