Fig. 8. Summary of MES-mediated histone methylation and models for the roles of the MES proteins in silencing the X chromosome in the germ line. (Left) The MES-2/MES-3/MES-6 complex catalyzes di- and trimethylation of H3K27, and preferentially concentrates H3K27me3 on the X chromosome (Bender et al., 2004). We hypothesize that MES-2/MES-3/MES-6 function repels MES-4 from the X chromosome and from regions of the autosomes (Fong et al., 2002) (this study). (Right) Two models for the role of MES-4. (Top right) MES-4 dimethylation of H3K36 in or near an autosomal gene (dotted line) activates that gene to express a repressor (labeled `R') of many X-linked genes. (Lower right) MES-4 concentrates H3K36me2 on the autosomes; MES-4 or H3K36me2, in turn, repels a repressor (labeled `R') from the autosomes, focusing its binding or action on the X chromosome. We speculate that the silencing of genes on the X chromosome is achieved by the combined repressive effects of H3K27me3 and repressor `R' action.