Spontaneous mutation arising on the C57BL/6 allele cis to the ApcMin allele confers resistance to intestinal polyp development compared to DBA/2J. A four base-pair duplication occurs in the coding region in exon 3. This causes a frameshift leading to the predicted aberrant incorporation of 42 amino acids in the protein product and a stop codon in exon 4. Real-time PCR analysis demonstrates significantly reduced levels of Atp5a1 mRNA in intestines of heterozygous mutant mice, and sequencing of the cDNAs detects none derived from mutant transcripts.