Testicular germ cell tumors (TGCT) originate from germ cells. The 129-Ter and M19 (129.MOLF-Chr19 consomic) mouse strains have extremely high incidences of TGCTs. We found that the expression levels of Sf1-encoded splicing factor 1 (SF1) can modulate the incidence of TGCTs. We generated mice with inactivated Sf1. Sf1 null mice (Sf1-/-) died before birth. Mice with one intact allele of Sf1 (Sf1+/-) were viable but expressed reduced levels of Sf1. When Sf1-deficient mice (Sf1+/-) were crossed to the 129-Ter and M19 strains, we observed decreased incidence of TGCTs in Sf1+/-;Ter and Sf1+/-;M19/+ mice compared with that in control cohorts. Therefore, Sf1 deficiency protects against TGCT development in both strains. Sf1 is expressed in the testes. We found that Sf1 levels vary significantly in the testes of inbred strains such as 129 and MOLF, and as such Sf1 is an oncogenic tumor-susceptibility factor from 129. Our results also highlight the complications involved in evaluating Sf1 levels and TGCT incidences. When a large number of tumor-promoting factors are present in a strain, the protective effect of lower Sf1 levels is masked. However, when the dosage of tumor-promoting factors is reduced, the protective effect of lower Sf1 levels becomes apparent. SF1 is involved in splicing of specific pre-mRNAs in cells. Alternate splicing generates the complex proteosome in eukaryotic cells. Our data indicate that Sf1 levels in mouse strains correlate with their incidences of TGCTs and implicate the importance of splicing mechanisms in germ cell tumorigenesis.