Previous studies from our laboratory demonstrated that the latency, tumor growth, and metastatic progression of polyoma middle T-induced mammary tumor in an FVB/NJ inbred mouse background could be significantly altered by the introduction of different genetic backgrounds. In this study we extend these findings by mapping a number of interacting quantitative trait loci responsible for the changes in phenotype. Introduction of the I/LnJ inbred genetic background into the FVB/NJ-PyMT animal significantly accelerated the appearance of the primary tumor (35 vs. 57 days postnatal, p < 10(-7)). A backcross mapping panel was established, and loci responsible for the tumor acceleration were detected on Chrs 15 and 9. Examination of the genotype/phenotype correlation revealed that the FVB/NJ but not the I/LnJ allele of the Chr 15 locus was associated with tumor acceleration and was conditional on the presence of I/LnJ allele on Chr 9. These loci, designated Apmt1 and Apmt2, map to homologous regions associated with LOH in human breast cancer. These results suggest that allelic variants of genes in these regions may contribute to age of onset in human breast cancer.