The aim of the present work was to obtain mutations in the genes involved in regulation of the yellow locus and mdg4. For this purpose, we searched for mutations changing phenotypic expression of the y(2) mutation induced by mdg4 insertion into the regulatory region of the yellow locus. Mutations have been obtained in the earlier described system of prolonged genome instability, sometimes combined with P-M hybrid dysgenesis. The mutation mod(mdg4) in a novel gene, modifier of mdg4, was detected which either enhanced or suppressed a phenotypic expression of several mutations induced by mdg4 insertion. We suggest that mod(mdg4) controls the expression of mdg4. In addition, the mutations in five loci located on the X chromosome have been found which enhanced the mutation phenotype of several y alleles induced by insertions of different mobile elements in the regulatory region of the latter. Possibly, the protein products of these genes designated as enhancers of yellow-1, 2, 3, 4 and 5 are directly or indirectly involved in control of the yellow locus expression.