A) PRC2 family of complexes. Core subunits are in lavendar and arrows depict association of optional subunits. Dashed lines indicate alternative subunits derived from multiple gene copies or protein variants from a single gene. B) and C) PRC1 family of complexes. B) Canonical PRC1 with four core subunits including a PC homolog (CBX in mammals). C) and D) PRC1 variants that contain KDM2 and/or RYBP subunits. In human PRC1 complexes, assembly of RYBP and CBX subunits are mutually exclusive. See (Gao et al., 2012; Gearhart et al., 2006; Lagarou et al., 2008; Tavares et al., 2012) for detailed descriptions of PRC1 variants. Ubiquitylation occurs on H2A-K119 in mammals, corresponding to K118 in Drosophila. E) PHO-RC and F) PR-DUB from Drosophila. Human homologs of PHO (YY1), SFMBT, ASX and Calypso (BAP1) exist and mammalian complexes containing ASXL1/2 and BAP1 have been described (Dey et al., 2012). Mammalian complexes comparable to fly PHO-RC have not been characterized.