Modifier msl1 mutations have reduced ability to recruit MOF ('males absent on first') and Male Specific Lethal (MSL)3 to the X chromosome in vivo. Polytene chromosomes from larvae of the genotype given on the left were treated with antibodies recognising the MSL subunits shown across the top. In all cases blue is DNA and red is the MSL staining. In cases where no MSL binding is evident, an arrow indicates the unstained X chromosome. (a)-(d) are males, (e) and (f) are female larvae expressing MSL2.