The Apc1638N mouse carries a targeted mutant allele at the endogenous adenomatous polyposis coli (Apc) gene and represents a unique in vivo model to study intestinal tumor formation and progression. Heterozygous Apc+/Apc1638N mice progressively develop 5-6 adenomas and adenocarcinomas of the small intestine within the first 6 months of life following a histologic sequence similar to that observed in human intestinal tumors. Here, we present the somatic mutation analysis of a total of 57 tumors. The results indicate that in ≤ 75% of the lesions tested the wild type copy of the Apc gene is lost and that this LOH event extends to the entire mouse chromosome 18. Unexpectedly, mutations at the K-, N- and H-ras genes have not been found in these tumors. Immunohistochemical analysis of the Apc1638N tumors failed to detect accumulation of the Tp53 protein. Also, no mutations have been found in exons 7 and 8 of the Tp53 gene. These results indicate that, although the genetic inactivation of Apc is involved in the initiating event of the human as well as murine intestinal tumorigenesis, tumor growth and progression follow different mutational pathways in these two species.