C57BL/6J (B6) mice containing the Mus domesticus poschiavinus Y chromosome, YPOS, develop ovarian tissue, whereas testicular tissue develops in DBA/2J or 129S1/SvImJ (129) mice containing the YPOS chromosome. To identify genes involved in sex determination, we used a congenic strain approach to determine which chromosomal regions from 129Sl/SvImJ provide protection against sex reversal in XYPOS mice of the C57BL/6J.129-YPOS strain. Genome scans using microsatellite and SNP markers identified a chromosome 11 region of 129 origin in C57BL/6J.129-YPOS mice. To determine if this region influenced testis development in XYPOS mice, two strains of C57BL/6J-YPOS mice were produced and used in genetic experiments. XYPOS adults homozygous for the 129 region had a lower incidence of sex reversal than XYPOS adults homozygous for the B6 region. In addition, many homozygous 129 XYPOS fetuses developed normal-appearing testes, an occurrence never observed in XYPOS mice of the C57BL/6J-YPOS strain. Finally, the amount of testicular tissue observed in ovotestes of heterozygous 129/B6 XYPOS fetuses was greater than the amount observed in ovotestes of homozygous B6 XYPOS fetuses. We conclude that a chromosome 11 locus derived from 129Sl/SvImJ essentially protects against sex reversal in XYPOS mice. A number of genes located in this chromosome 11 region are discussed as potential candidates.