The suppressor of Hairy-wing [su(Hw)] protein inhibits the function of transcriptional enhancers located distal to the promoter with respect to the location of su(Hw)-binding sites (insulation). Mutations in the modifier of mdg4 [mod(mdg4)] interfere with insulation and enhance the effect of the su(Hw)-binding region inserted in the y2 mutation by inhibiting the function of regulatory elements located on both sides of the su(Hw)-binding region. From P-M hybrid dysgenic crosses, 21 mutations that suppress the negative effect of the mod(mdg4)1u1 mutation on the y2 allele were obtained among 47,000 flies scored. These Su(mg) mutations have a dominant suppressor effect and map to at least 13 different loci. Some of Su(mg) mutations also suppress the effect of mod(mdg4)1u1 on two other gypsy-induced mutations, scD1 and ct6. Most of Su(mg) mutations do not affect the viability or fertility of homozygous flies. We speculate that the Su(mg) genes represent a new family of redundant regulatory genes in Drosophila melanogaster.