Mutations in msl1 abolish local Male Specific Lethal (MSL) spreading. In all cases except (c), polytene chromosomes were taken from males carrying one copy of GMroX1-75C inserted on the third chromosome (arrow). In (c) the transgene is homozygous. Chromosomes were stained with anti-MSL1 antibodies (red). In (a)-(c) the X chromosome is roX1ex6 roX2+. (a) msl1+/msl1+, (b) and (c) msl1F979Z/msl1+. In (d)-(i) the X chromosome is roX1ex6 roX252. (d) msl1+/msl1+ the MSL complex covers 73A-77C, (e) msl1+/msl1L60, (f) msl1+/msl1K1009X, (g) msl1+/msl1F979Z, P864L, AFG and K1009X also fail to support local MSL spreading (not shown), (h) mle+/mlenull, the mleI926Z allele was also examined and gave identical results (not shown), (i) msl2+/msl2null. Dense MSL1 staining occurs over the X chromosome.