ATM and ataxia telangiectasia: It was found to be homozygous in an individual with ataxia-telangiectasia (A-T) and heterozygous in an individual affected with breast cancer.54,55 An experimental study that used a lymphoblastoid cell line derived from an A-T affected individual has shown that this missense change causes a defect in RNA splicing with complete loss of the ATM protein.