A modified P element-induced mutation at cytological position 47A11-12 on chromosome 2 of Drosophila melanogaster inversely alters the expression of several genes examined. In addition, the effect is nearly exponential in some developmental stages for five of the eleven genes tested (white, brown, scarlet, rudimentary and vermilion). Interestingly, the exponential overexpression of the transcripts leads to a sexually dimorphic effect, with female levels being greater. The white allele specificity analysis and the lack of response of an Alcohol dehydrogenase promoter-white reporter fusion construct suggest that the 5' regulatory sequences of white are required for the response. Because the cases of extreme overexpression are more prominent in females than males, the locus is termed Ultra female overexpression (Ufo). Ufo revertants have no discernible effect on the transcript levels, indicating that the mutation is responsible for the modulation of the tested genes. Regulatory genes such as Ufo may contribute to the wide fluctuations in gene expression between tissues and stages of development.