The binary switch gene Sex-lethal ( Sxl ) controls sexual identity in Drosophila . When activated, Sxl imposes female identity, whereas male identity ensues by default when the gene is off. The decision to activate Sxl is controlled by an X chromosome counting system that regulates the Sxl establishment promoter, Sxl-Pe . The counting system depends upon the twofold difference in the gene dose of a series of X-linked transcription factors or numerators. Because of this difference in dose, early female embryos express twice the amount of these transcription factors, and the cumulative action of these transcription factors turns on Sxl-Pe . Here we show that the Drosophila Myc gene diminutive is an X-linked numerator.