All cancer types exhibit familial clustering, suggestive of a significant inherited component; however, to date only a few of the genes responsible have been identified and the inherited component, if any, underlying most common cancers has not been well defined. Amongst the important known susceptibility genes are those dominant genes conferring a high risk of breast and ovarian cancer (BRCA1), colon cancer (hMSH2 and hMLH1), and melanoma (MLM). All these genes confer a high lifetime risk of the disease concerned, but are rare and only account for a small minority (less than 5%) of cases. However, there are also commoner genes conferring lower risks but accounting for a more substantial fraction of cancer cases; those so far identified include the ataxia-telangiectasia gene and the HRAS1 minisatellite locus.