A recent paper in PLoS Biology looks at the genome-wide trimethylation pattern of histone H3 lysine 27 (H3K27me3) in Arabidopsis, discovering that it is a primary gene silencing mechanism. Using ChIP-chip, researchers in the Jacobsen lab at UCLA identified for the first time H3K27me3-associated regions across the entire genome of the flowering plant, finding that trimethylation regulates an unexpectedly high 4,400 genes.