For example, ATM missense mutations are more common than ATM null mutations in breast cancers (22, 23), and mice carrying a copy of a mutated Atm allele develop more tumors than mice heterozygous for a silenced Atm allele (Atm±) (24), suggesting the existence of mono-allelic, dominant negative mutations in tumor suppressors. The gene discussed is ATM; the disease is breast carcinoma.