Required, together with Trl, for maintaining the repressed state of target genes including homeotic genes Scr and Ubx. May also be involved in the activation of homeotic genes. Binds to a DNA Polycomb response element (PRE) at the bithorax complex. Also binds to polytene chromosomes at several hundred sites, many of which are shared with Trl and ph-p. Required during embryonic development. The BTB domain interacts with the BTB domain of Trl in vitro. Found in a Pc-containing complex.