Modifiers of dosage compensation. (a) From approximately 10,000 G1 sons screened, 13 modifier lines were established. (b) The mutations were placed in complementation groups by assaying for recessive lethality. Alleles of groups A and B are indicated by arbitrary isolation numbers and are lethal to both males and females. Alleles of msl1 and mle are named by their codon changes where Z = frameshift and X = nonsense codon. *The modifiers scored as single hits each carries a recessive lethal mutation, but it is not known whether this maps to the modifier locus or an extraneous gene on the same chromosome. (c) The hemizygous GMroX1-75C males displays mosaic pigmentation when the endogenous roX+ genes are present and the Male Specific Lethal (MSL) complex binds a single band at 75C. Male eye colour is derepressed and MSL complex spreads > 1 Mb when both endogenous roX genes are deleted. (d) All flies are homozygous for GMroX1-75C and heterozygous for the msl1 mutation indicated. The top row shows males, the lower two rows are females. The mosaic eyes of GMroX1-75C homozygous males become almost solid red when a modifier mutation is present. Mutations P864L, F979Z and S943F are new alleles of msl1 that were isolated in the present study. Df = msl1L60  has no effect on the mosaic pattern. AFG and K1009X are previously identified msl1 lesions that map near the modifier mutations, but have little or no effect on the mosaic miniwhite expression. Bottom rows: GMroX1-75C females have pure white eyes and this is not affected by any of the modifier mutations. (e) Homozygous P864L escaper males display enhanced miniwhite expression. (f) The modifier mutants (msl1P864L shown) have no effect upon In(1) wm4 whereas Su(var)2-5 is a potent suppressor of position effect variegation (PEV).