ATM and ataxia telangiectasia: However, this study did not distinguish between the effects of protein-truncating and missense mutations, although Gatti et al. [15] had hypothesised in 1999 that, compared with protein-truncating mutations, some missense variants in ATM might act as dominant negatives and confer a particularly high risk of breast cancer when heterozygous, although causing a milder form of AT when homozygous.