The spontaneous development of juvenile-onset ovarian granulosa cell tumors in mice of the SWXJ-9 recombinant inbred strain is a model for juvenile-type granulosa cell tumors that appear in very young girls. To expedite gene discovery in this mouse model of childhood cancer, we did a gene mapping study with the SWXJ-9 recombinant inbred strain and the evolutionarily divergent Mus musculus castaneus (CAST/Ei) strain as a mapping partner. Our mapping strategy focused on autosomal determinants of susceptibility with a backcross scheme that exploited a paternal, parent-of-origin effect for a X-linked gene (Gct4) that strongly supports granulosa cell tumor development. Of 1,968 backcross females examined, we detected 81 granulosa cell tumor-bearing animals and compared their allelic inheritance patterns to non-tumor-bearing siblings in a case-control analysis. The results of our study have confirmed an important locus on mouse chromosome (Chr) 4 (Gct1) and have revealed new loci for granulosa cell tumor susceptibility (Gct7-Gct9) on Chrs 1, 2, and 13 with susceptibility alleles contributed by the SWXJ-9 progenitor. Two novel gene-gene interactions supportive for granulosa cell tumor development were also observed between loci on Chrs 17 and 18 and loci on Chrs 2 and 10. Our data substantiate the evidence that Gct1 on Chr 4 is a fundamental oncogene for granulosa cell tumorigenesis in mice and has identified additional interacting autosomal loci that support tumor development.