The colony of wild mice, named Jyg, has been shown to express an exogenous mouse mammary tumor virus (Jyg-MMTV). This virus induces mammary tumors in its natural host at a high incidence ( approximately 80%) resulting from insertion mutations in Notch-4 (43%), Wnt-1 (26%), and Fgf-3 (13%). Since the activation of Notch-4 is not common in mammary tumors of standard laboratory strains of mice infected with various MMTV strains, we examined the consequences of Jyg-MMTV infection in BALB/c and C57BL/6 mice. The results show that Jyg-MMTV induces mammary tumors in both mouse strains, but the incidence of mammary tumors in BALB/c mice is greater than in C57BL/6 mice. Surprisingly, however, none of the 75 mammary tumors, analyzed both by Southern and Northern hybridizations, showed insertion mutations in or expression of Notch-4. In contrast, both Wnt-1 and Fgf-3 were found to be involved in these tumors. Our findings may suggest, among other possibilities, the existence of a structural difference(s) between laboratory and wild mice at the Notch-4 locus that regulates the integration of Jyg-MMTV proviral DNA.