Somatic mutations in the tumor suppressor gene p53 occur in more than half of all human cancers. Rare germline mutations result in the Li-Fraumeni cancer family syndrome. In this issue of Genes & Development, Jennis and colleagues (pp. 918-930) use an elegant mouse model to examine the affect of a polymorphism, P47S (rs1800371), in the N terminus of p53 that is found in Africans as well as more than a million African Americans. Remarkably, the single nucleotide change causes the mice to be substantially tumor-prone compared with littermates, suggesting that this allele causes an increased risk of developing cancer. The defect in p53 function is traced to a restriction in downstream gene regulation that reduces cell death in response to stress.Li-Fraumeni syndrome, which is caused by germline mutations in the p53 gene, has been extensively studied. It is associated with an extraordinarily high risk of cancer development of various types, including pediatric sarcomas and brain cancers as well as very early onset female breast cancer (Malkin et al. 1992). The severity of the syndrome is related to the exact nature of the mutation in p53, with missense mutations in the DNA-binding domain predominating. Interestingly, a large cohort of individuals in Brazil that harbor a mutation in the C terminus of p53 show a much milder phenotype, generally presenting with adrenocortical tumors as young adults. Mouse models of LiFraumeni syndrome reflect these features. Using the hybrid mouse/human Hupki allele of p53 (Luo et al. 2001), a range of p53 point mutations has been tested for their effects on tumor incidence and spectrum in mice. Broadly, the results show that missense mutations in p53 result in higher tumor incidence in mice, although the type of tumors that develop is strongly dependent on strain background. This finding emphasizes the importance of modifier genes in the mechanisms of p53 tumor suppression (Goh et al. 2011). Indeed, polymorphic variations in the promoter of Mdm2, a p53 target gene and negative regulator, show effects on tumor incidence in both mouse models and Li-Fraumeni cohorts, thereby defining it as a modifier gene (Pietsch et al. 2006;Grochola et al. 2010).Further analysis suggests a difference between p53 alleles based on the exact nature of the mutation. For example, some alleles show a dominant-negative phenotype, while others also show a gain of function. In these cases, the mutant protein induces phenotypes like invasion and metastasis that differ from the effects of simple loss of function (Muller and Vousden 2014). These findings, along with clear cell biological studies that demonstrate that p53 is cell-autonomous and haploinsufficient for cell death responses to radiation, suggest that the precise level and biochemical nature of the p53 protein can profoundly modify its tumor suppressor activity. The potential significance of known p53 polymorphisms in the human population is thus a matter of great interest, as these variations in p53 may affect an individual's risk of developing cancer...