The c.4158dup (p.Lys1387*) variant in ATM is a nonsense variant predicted to cause a premature stop codon in 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 two unrelated individuals with Ataxia-Telangiectasia (PMID: 26896183). The highest minor allele frequency in gnomAD v4.1.0 is 0.00003381 (1/29576 alleles) in the Ashkenazi Jewish population; while this is higher than the HBOP VCEP threshold (≤0.00001) for PM2_Supporting, it is present in only one allele, meeting this criterion. In summary, this variant meets the 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, PM5_Supporting, PM3_Strong, PM2_Supporting)