How does the promoter of an oocyte-specific gene function in male germ cells?

Studying gene expression in germ cells is useful for elucidating mechanisms of transcriptional regulation, because different genes are activated in male and female germ cells. The promoter regions of an oocyte-specific gene, Oog1, have been characterized. Driving the expression of green fluorescent protein with these different promoter regions provided us with critical information on the regulation of gene expression. The 3.9 kb long promoter functions in both male and female germ cells in transgenic mice. What is the cause of this sexually dimorphic expression? There may be important factors within and perhaps also outside this 3.9 kb promoter region that are required to maintain proper sex-specific gene expression.