Genome-Wide H3K9m2 Profiling Figure 3. Relationship between H3K9m2 levels and the length of H3K9m2 positive regions in different chromosomal compartments. (a) Examples of typical H3K9m2 levels present in either euchromatic chromosomal arms or pericentromeric regions. Each gray bar corresponds to the Z-score of an individual probe. (b) Distribution of z-scores in the euchromatic arms (green) and pericentromeric regions (red). (c) Lengths of uninterrupted H3K9m2 methylated regions, with each horizontal bar representing a single region in either the euchromatic arms or pericentromeric regions. doi:10.1371/journal.pone.0003156.g003 more efficiently replicate epigenetic states during chromatin replication, whereas smaller elements in the chromosome arms may require more active targeting by siRNA mediated processes because they are too small for efficient transmission of epigenetic information between adjacent nucleosomes [38]. This more active targeting may also explain the higher levels of CHG DNA methylation seen in euchromatic arms. Regardless of the mechanism, these findings underscore a possible difference in PLoS ONE | www.plosone.org 5 September 2008 | Volume 3 | Issue 9 | e3156