p27^Kip is a candidate human tumour-suppressor protein, because it is able to inhibit cyclin-dependent kinases and block cell proliferation^,,,,. Abnormally low levels of the p27 protein are frequently found in human carcinomas, and these low levels correlate directly with both histological aggressiveness and patient mortality^,,,,. However, it has not been possible to establish a causal link between p27 and tumour suppression, because only rare instances of homozygous inactivating mations of the p27 gene have been found in human tumours^,,,. Thus, p27^Kip1 does not fulfil Knudson's ‘two-mutation’ criterion for a tumour-suppressor gene. Here we show that both p27 nullizygous and p27 heterozygous mice are predisposed to tumours in multiple tissues when challenged with γ-irradiation or a chemical carcinogen. Therefore p27 is a multiple-tissue tumour suppressor in mice. Molecular analyses of tumours in p27 heterozygous mice show that the remaining wild-type allele is neither mutated nor silenced. Hence, p27 is haplo-insufficient for tumour suppression. The assumption that null mutations in tumour-suppressor genes are recessive excludes those genes that exhibit haplo-insufficiency.