Fig. 3. Mapping the genome-wide distribution of Polycomb group (PcG) proteins in mouse and human embryonic stem (ES) cells. PcG proteins form well-known biochemical complexes: one is PCR2, which contains the H3K27-specific histone methyltransferase EZH2, and its two partners EED and SUZ12; the second complex, named PRC1, contains chromodomain mouse homologues of Drosophila Polycomb, which bind to trimethylated H3K27 and to the two interacting proteins PHC1 and RNF2. (A) A Venn diagram represents the number of genes bound by each of these components and their overlap, showing extensive colocalization among all members tested (Boyer et al., 2006). (B) A similar mapping for human embryonic stem cells identified a large number of transcription factor-coding genes that can be grouped into major developmental transcription factor families (Lee et al., 2006). This illustrates how PcG proteins regulate a variety of crucial developmental patterning processes. Figure courtesy of Rudolf Jaenisch (Whitehead Institute for Biomedical Research and Massachussets Institute of Technology, Cambridge, MA, USA).