The CDKN2b-CDKN2a locus on chromosome 9p21 in human (chromosome 4 in mouse) is frequently lost in cancer. The locus encodes three cell cycle inhibitory proteins: p15(INK4b) encoded by CDKN2b, p16(INK4a) encoded by CDKN2a and p14(ARF) (p19(Arf) in mice) encoded by an alternative reading frame of CDKN2a (ref. 1). Whereas the tumour suppressor functions for p16(INK4a) and p14(ARF) have been firmly established, the role of p15(INK4b) remains ambiguous. However, many 9p21 deletions also remove CDKN2b, so we hypothesized a synergistic effect of the combined deficiency for p15(INK4b), p14(ARF) and p16(INK4a). Here we report that mice deficient for all three open reading frames (Cdkn2ab(-/-)) are more tumour-prone and develop a wider spectrum of tumours than Cdkn2a mutant mice, with a preponderance of skin tumours and soft tissue sarcomas (for example, mesothelioma) frequently composed of mixed cell types and often showing biphasic differentiation. Cdkn2ab(-/-) mouse embryonic fibroblasts (MEFs) are substantially more sensitive to oncogenic transformation than Cdkn2a mutant MEFs. Under conditions of stress, p15(Ink4b) protein levels are significantly elevated in MEFs deficient for p16(Ink4a). Our data indicate that p15(Ink4b) can fulfil a critical backup function for p16(Ink4a) and provide an explanation for the frequent loss of the complete CDKN2b-CDKN2a locus in human tumours.