Transmission ratio distortion (TRD) and sterility are male-specific quantitative trait phenomena associated with the mouse t haplotype. TRD occurs in t haplotype-heterozygous males and is caused by the deleterious action of distorter products on sperm bearing a wild-type responder locus. It has been proposed that t-mediated male sterility is a severe manifestation of TRD caused by homozygosity for distorter loci; thus, the distorter and sterility loci would be identical. In this, study a transgenic approach was used to identify the proximal sterility locus, tcs1 (S1), and test its role in TRD. Mice transgenic for a wild-type bacterial artificial chromosome (BAC) derived from the S1-critical region were bred onto t haplotype backgrounds. Mating results conclusively showed that the BAC is sufficient to restore fertility in otherwise sterile males. Multiple mutations were identified in the t alleles of Synj2 and Serac1, two genes in the BAC; thus, they are candidates for S1. In addition, whereas the BAC transgene rescued sterility, it had no effect on TRD. These results uncouple the proximal t haplotype sterility locus, S1, from TRD, demonstrating that S1 and the proximal distorter locus, D1, are not the same gene.