Strain variation in the mouse p53 gene sequences was investigated in various regions of the gene in 14 inbred strains of laboratory mice and one Japanese wild mouse strain (Mus musculus molossinus Mishima, M. MOL-MSM). Nucleotides within p53 introns 1 and 7, found to be identical in 10 of the laboratory strains (129/J, A/J, AKR/J, BALB/cJ, C3H/HeJ, C57BL/6J, CBA/J, CE/J, NZB, and SWR/J), were substituted for other nucleotide sequences in common with M. MOL-MSM and the four other strains (DBA/1J, DBA/2J, I/LnJ, and P/J). The latter were documented to have originated from a common ancestor. These observations thus suggested the possibility that the p53 gene may have become substituted by outcrossing of this ancestral strain with Asian mice; this is presumably related to the documentation that Japanese mice brought to western countries were used as laboratory mice early in this century. To establish p53 gene heterozygosity, female C3H/HeJ and male DBA/2J mice were mated to produce F1 hybrids (C3D2F1). Electrophoresis of PCR fragments including polymorphic regions with or without restriction enzyme digestion, allowed clear distinction of paternal and maternal p53 alleles. These markers, therefore, should be useful for studying the loss of heterozygosity of the p53 gene during the carcinogenic process.