The search for the relevant 129P2 allele(s) involved in the skin cancer phenotype of Cdkn2ab KO mice was started by generating chimeric mice using the 129P2 ES cell clone carrying knockout alleles for p15Ink4b and p16Ink4a and conditionally knockout for p19Arf (ref. 2). The gene discussed is CDKN2A; the disease is skin cancer.