The Polycomb and trithorax group genes encode trans-regulators of homeotic gene function in Drosophila. The Polycomb group genes encode transcriptional repressors, while the trithorax group proteins are positive factors required for homeotic gene function. Among the Polycomb group proteins, the POLYCOMB protein has been most extensively characterized. The POLYCOMB protein contains a chromodomain, a conserved domain found in a Drosophila protein with effects on position-effect variegation. Among the trithorax group proteins characterized, the BRAHMA protein appears to be a subunit of a protein complex conserved from yeast to man (the SNF/SWI complex) that modifies chromatin to facilitate the transcriptional activation by gene-specific DNA-binding proteins. The ZESTE protein may help to activate transcription by bringing distant cis-regulatory elements closer to promoter-bound proteins.