The p53 knockout mouse has been widely used as a model in cancer research and other applications. Because neither homozygous nor heterozygous mutant p53 mice exhibit an overt phenotype, each animal requires laborious molecular genotyping. Here we describe a new p53 mutant mouse that is tagged with a tyrosinase coat color minigene. On an albino background, heterozygous tyrosinase-tagged p53 mutant mice exhibit a light tan coat color, while homozygous mutants display a darker brown coat color. Thus, by 8-10 days of age, mice with two, one, or no mutant p53 alleles are immediately distinguishable by their coat color, eliminating the time, costs, and errors associated with molecular genotyping. Moreover, the homozygous mutant p53-tyrosinase mice display a tumor incidence and spectrum virtually identical to previous p53 null mouse lines. Thus, tagging targeted mutations with such coat color markers provides a generally applicable genotyping method for embryonic stem cell-derived mice.