This is the case for mutations in the PALB2, BRIP1 and ATM genes, where heterozygotes have an increased lifetime risk of breast cancer, and homozygotes are diagnosed with multisystemic genetic syndromes (Fanconi’s anaemia for the first 2 genes and ataxia-telangiectasia for the latter). The gene discussed is BRIP1; the disease is Ataxia-telangiectasia.