We used a system with a mobilized Stalker transposable element, sometimes in combination with P-M hybrid dysgenesis, in the search for new mutations interfering with the y2 mutation induced by mdg4 (gypsy) insertion into the yellow locus. A novel gene, modifier of mdg4, was detected in chromosome 3. The mutation mod(mdg4) either enhanced or suppressed phenotypic changes in different mutations induced by mdg4 insertions. Thus, mod(mdg4) seems to be involved in the control of mdg4 expression. Six other loci designated as enhancers of yellow were also detected. The e(y)n (with n from 1-6) mutations enhanced the expression of several y mutations induced by different insertions into the yellow locus. The major change is a damage of bristle and hair pigmentation which is not suppressed by su(Hw) mutations. On the other hand, e(y)n alleles do not interact with mdg4 induced mutations in other loci. All e(y)n genes are located in different regions of the X chromosome. One may speculate that e(y)n genes are involved in trans-regulation of the yellow locus and possibly of some other loci.