It has been postulated and supported by limited experimental evidence that the presence of ATM missense mutations, in contrast to ATM truncating mutations, may result in a different cellular phenotype after exposure to IR and alter the risk of developing cancers in heterozygote carriers (Stankovic et al, 1998; Gatti et al, 1999; Chenevix-Trench et al, 2002; Scott et al, 2002). This evidence concerns the gene ATM and cancer.