The transcription factor Runt can repress expression of the Drosophila segment-polarity gene engrailed (en) in the odd-numbered stripes of expression seen in the developing Drosophila embryo. In an Advanced Online Publication in Nature Genetics John Wheeler and colleagues describe a genetic screen for factors that potentiate Runt activity (Nat Genet 2002, DOI:10.1038/ng942).

Wheeler et al. identified genomic regions that dominantly suppress Runt-induced lethality and identified associated genes. This screen led them to four genes that encode repressors: dCtBP, Groucho, Rpd3 and tramtrack (ttk). Analysis of engrailed expression in the different genotypes led them to a two-step model for Runt repression. Initiation of repression by Runt requires co-operation with Ttk, whereas maintenance after the blastoderm stage requires interaction with dCtBP, Groucho and Rpd3.