ABSTRACT:The regulation mechanism of female-predominant expression of the mouse Cyp2b9 gene was investigated in vivo and in vitro. Luciferase reporter assay revealed that the ؊234/؊194 region of the Cyp2b9 gene may be responsible for sexually dimorphic expression. There is a predicted forkhead box A2 (FoxA2) (hepatic nuclear factor 3␤)-binding site in this region. Chromatin immunoprecipitation assay indicated that the binding protein to the site was FoxA2 in 5-week-old female mice, whereas this protein was found in both sexes at age 3 weeks, in accordance with our previous observation on the developmental expression of this gene.Mutation of the predicted FoxA2 site in the reporter construct containing the ؊234/؉18 fragment led to complete elimination of luciferase activity, but deletion of the ؊234/؊194 region resulted in considerable transcriptional activity, suggesting that by mutating the FoxA2-binding site a potent suppressor might bind to eliminate activity, whereas by deleting this region it could not. Sexually dimorphic secretion of growth hormone is involved in femalepredominant expression of the gene, and the ؊234/؊194 region was also responsible for suppressing the expression by male-type secretion.