The c.8480T>G variant in ATM is a missense variant predicted to cause the substitution of phenylalanine by cysteine at amino acid 2827 (p.Phe2827Cys). This variant has been detected in at least one individual with Ataxia-Telangiectasia, who was described having a mild presentation (PMIDs: 8755918, 9000145, 9463314, 15928302, 19431188, 25040471, 30549301). The highest population minor allele frequency in gnomAD v4.1 is 0.00002196 in the South Asian population (PM2_Supporting, BS1, and BA1 are not met). Stable transfection of ATM-based cDNA constructs in an ATM-null lymphoblastoid cell line showed slightly decreased levels of kinase activity (caused by protein instability) indicating that this variant impacts protein function (PMID 19431188). The computational predictor, REVEL, gives a score of 0.889, which is above the threshold of 0.7333, evidence that correlates with impact on ATM function. In summary, this variant meets the criteria to be classified as likely 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. (PS3_Supporting, PM3_Strong, PP3)