Fig. 1. RNAi and RNA-directed DNA methylation (RdDM) are both triggered by dsRNA. dsRNAs are cleaved by enzymes of the Dicer family to generate small, 21-26 nucleotide siRNAs. These in turn are taken up by the RNA-induced silencing complex (RISC) to direct degradation of complementary RNA sequences. Such RNAs are also involved in RdDM, which is mediated by three classes of methyltransferase: DRM, CMT and MET1.