In mice heterozygous for p53 (Trp53+/−), the incidence of mammary tumors varies among strains, with C57BL/6 being resistant and BALB/c being susceptible. Mammary tumor phenotypes were examined in female Trp53+/− F1 mice (C57BL/6 × BALB/c;n = 19) and N2 backcross mice [(C57BL/6 × BALB/c) × BALB/c] (n = 224). Susceptibility to mammary tumors segregated as a dominant phenotype in F1 females, but a higher frequency and shorter latency in N2 mice indicated a contribution from recessive-acting modifiers. Segregation of the hypomorphic BALB/c alleles for DNA-dependent protein kinase catalytic subunit (Prkdc) and p16INK4A (Cdkn2a) was analyzed in the N2 mice. The time to first tumor (considering all tumor types) was significantly different among the four genotype combinations (P = 0.01). Cdkn2a had a strong effect (P = 0.008) but was restricted to PrkdcB/B mice (P = 0.001), indicating a strong interaction between the loci. Differences in mammary tumor occurrence among genotypes for Prkdc and Cdkn2a in N2 mice were not statistically significant. This study indicates that BALB/c Prkdc and Cdkn2a alleles do modify tumor incidence in Trp53+/− mice and highlights the complexity of gene interaction effects in determining cancer phenotypes but discounts these alleles as major recessive loci contributing to spontaneous mammary tumor susceptibility.