Fig. 7. Model of PcG complex assembly at Hox target loci. Nucleosomes and methylated H3-K27 are depicted as blue ovals and orange diamonds, respectively. Clustered YY1 binding sites are indicated in red. The purple line represents potential cooperative interactions between RNA molecules and PcG proteins. Based on previous PcG and trxG mutant analysis (Yu et al., 1995; Yu et al., 1998; Deschamps et al., 1999; Tomotsune et al., 2000), the transition from initiation to maintenance of Hox gene expression should occur between E9 and E10 of mouse development, herein referred to as `Early Maintenance' phase. Complex assembly during the `Late Maintenance' phase was ascertained from E12.5 embryos.