Expression is first observed at 11.0 dpc, and is prominent at 11.5 dpc, the sex-determining stage of gonad development. Highest NPC 15199 levels of expression at this stage are detected in the coelomic epithelium and sub-epithelial mesenchymal cells. The significance of this profile is unclear, although the coelomic epithelium is the source of at least a subset of Sertoli cells and male-enhanced cell proliferation in this region is one of the first consequences of SRY expression. Reporter signal is diminished by 13.5 dpc and is restricted to the coelomic epithelium. This observation is in contrast to Map3k1 in situ hybridisation analysis, which reveals expression in the ovary and testis cords at 13.5 dpc, consistent with expression in pre- Sertoli cells. This discrepancy might be explained by the loss of regulatory elements in the targeted Map3k1DKD allele. Other aspects of the in situ and reporter expression profiles, including expression in the developing reproductive tracts, are in agreement, suggesting that the probe utilised for in situ hybridisation is of good quality. One abnormality that was observed in Map3k1DKD homozygous embryos was an increased length of mutant gonads at 13.5 dpc. The cellular basis for this phenotype is unclear, however, it might explain the presence of small clusters of Teijin compound 1 Stra8-positive cells at the caudal pole of some mutant gonads. The program of testis development is thought to radiate from initial SRY-dependent events in the centre of the gonadal primordium. This centre-topole masculinisation is thought to be driven by the secreted molecule, FGF9. Any delay in the receipt of this signal by the gonadal poles can result in the formation of ovarian tissue in this region, as characterised by ovotestis development, due to antagonism of the testis-determining pathway by ovarian-determining genes. The increase in the length of the mutant gonads reported here might result in occasional delay in the masculinising signal reaching the poles and subsequent entry of XY primordial germ cells into meiosis. Notably, successful testis determination in Map3k1DKD homozygotes is not overtly disrupted by the further removal of a single copy of Map3k4, a known dosage-sensitive sex-determining gene in the mouse that also functions in the MAPK signalling pathway to activate JNK and p38. Thus, we conclude that MAP3K1 is not required for testis determination in the mouse, potentially highlighting a difference between mice and humans with respect to MAP3K1 and its role in sex determination. What might account for this apparent discrepancy?