The c.8565_8566delinsAA variant in ATM is an in-frame deletion insertion predicted to cause substitution of serine and valine by arginine and isoleucine at amino acids 2855-2856 This variant has been detected in multiple unrelated individuals with Ataxia-Telangiectasia (PMIDs: 9872980, 10817650, 12673797, 26896183, 37438524). This variant is absent from gnomAD v4.1.0. Western blotting in ATM null cells transfected with cDNA carrying this variant showed inactive phosphorylation of ATM downstream targets as compared to wild-type controls indicating that this variant impacts protein function (PMID: 19431188). The computational predictor Provean gives a score of -4.871, evidence that correlates with impact to ATM function. 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. (PM3_Very Strong, PM2_Supporting, PS3_Supporting).