The chromodomain (CD) of the Drosophila Polycomb protein exhibits preferential 
binding affinity for histone H3 when trimethylated at lysine 27. Here we have 
investigated the five mouse Polycomb homologs known as Cbx2, Cbx4, Cbx6, Cbx7, 
and Cbx8. Despite a high degree of conservation, the Cbx chromodomains display 
significant differences in binding preferences. Not all CDs bind preferentially 
to K27me3; rather, some display affinity towards both histone H3 trimethylated 
at K9 and H3K27me3, and one CD prefers K9me3. Cbx7, in particular, displays 
strong affinity for both H3K9me3 and H3K27me3 and is developmentally regulated 
in its association with chromatin. Cbx7 associates with facultative 
heterochromatin and, more specifically, is enriched on the inactive X 
chromosome. Finally, we find that, in vitro, the chromodomain of Cbx7 can bind 
RNA and that, in vivo, the interaction of Cbx7 with chromatin, and the inactive 
X chromosome in particular, depends partly on its association with RNA. We 
propose that the capacity of this mouse Polycomb homolog to associate with the 
inactive X chromosome, or any other region of chromatin, depends not only on its 
chromodomain but also on the combination of histone modifications and RNA 
molecules present at its target sites.