The c.1402_1403del (p.Lys468Glufs*18) variant in ATM is a frameshift variant in a biologically-relevant-exon predicted to cause a premature stop codon 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-terminus 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 3 individuals with Ataxia-Telangiectasia (PMID: 23807571, 26896183, 31691010). The highest population minor allele frequency in gnomAD v2.1.1 is 0.0003 in the East Asian population (PM2_Supporting, BS1, and BA1 are not met). 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 Variant Curation Expert Panel. (PVS1, PM5_Supporting, PM3_Strong)