In XY male, the proteins of male-specific lethals (MSLs) bind to gene-rich regions of the X chromosome to increase gene expression in male flies. On the other hand, Sxl is active in XX female and functions to inactivate one or more of the male-specific autosomal genes, mle. By dampening the activity of these autosomal genes, it ensures that they will not serve to double the expression of X-linked genes in female.