The multifactorial aetiology of colorectal cancer involves environmental factors as well as genetic suscep-

of the disease worldwide. For the majority of individuals, an ill-defined increased risk of the disease is indicated by having an affected relative with around 20-25 per cent of all colorectal cancer cases being associated with a family history of the disease.1 Screening studies suggest that cancer susceptibility is due to predisposition to the development of colorectal adenomas.2 Fortunately, not all adenomas progress to cancer because the population frequency of the ‘adenoma-prone’ allele was calculated as 19 per cent. The effects of diet clearly must influence the expression of such an allele in terms of both adenoma and cancer, but the trait could probably be thought of as a ‘normal’ variation of the human constitution. Perhaps the most radical suggestion is that colorectal neoplasms only occur in the presence of a genetic predisposition.2