The PEV-modifying winged-helix/forkhead domain transcription factor JUMU of Drosophila is an essential protein of pleiotropic function. The correct gene dose of jumu is required for nucleolar integrity and correct nucleolus function. Overexpression of jumu results in bloating of euchromatic chromosome arms, displacement of the JUMU protein from the chromocenter and the nucleolus, fragile weak points, and disrupted chromocenter of polytene chromosomes. Overexpression of the acidic C terminus of JUMU alone causes nucleolus disorganization. In addition, euchromatic genes are overexpressed and HP1, which normally accumulates in the pericentric heterochromatin and spreads into euchromatic chromosome arms, although H3-K9 di-methylation remains restricted to the pericentric heterochromatin. The human winged-helix nude gene shows similarities to jumu and its overexpression in Drosophila causes bristle mutations.

Gene expression goes along with changes in chromatin structure and is regulated by chromatin-modifying factors. If genes are transposed from their euchromatic position to the vicinity of heterochromatin, their expression can underly a position effect variegation (PEV). In Drosophila melanogaster a few genes are known that function in a gene dose-dependent manner as haplo-suppressors and triplo-enhancers of PEV or vice versa. The gene jumeaux (jumu) encodes a winged-helix transcription factor of multiple regulatory functions. A novel PEV test system for Drosophila melanogaster reveals that JUMU behaves as a haplo-suppressor/triplo-enhancer in different larval and adult tissues, but surprisingly behaves in the reverse manner as a haplo-enhancer/triplo-suppressor in larval and adult brains. Like jumu, the Su(var)3-9 gene also behaves as a haplo-suppressor/triplo-enhancer, but in our test system does not show any PEV effect in the brains.