To test the hypothesis that genetic instability in the p53 locus stage might be a key early event in tumorigenesis, with change in the stroma possible influencing epithelial tumorigenesis, microsatellite markers D17S796 (Gyapay et al, 1994), TP53 and D17S786 (Gyapay et al, 1994), located in the short arm of chromosome 17 within 4 cM proximity to the p53 gene (17p13), and D17S579 (17q21) (Anderson et al, 1993) in the long arm of the chromosome 17 were analysed here in a series of sporadic colorectal cancers. The gene discussed is TP53; the disease is colorectal cancer.