Sry is the testes determining factor gene located on the Y chromosome. We sought sequence variation in potential promoter regions that might explain the gradiation in sex reversal seen when different domesticus strains are bred to C57BL/6J females since sequence variation previously found in the coding region is not sufficient to explain it. Dideoxy sequencing identified a 10 bp deletion in the 5' flanking region limited to YDOM chromosomes of weak effect. A PCR strategy to search for deletions in this region disclosed another deletion which was shown to be different by sequencing.