RNA polymerase IV (RNAP IV) is an enzyme that synthesizes small interfering RNA (siRNA) in plants, which silence gene expression.[1][2][3] RNAP IV belongs to a family of enzymes that catalyze the process of transcription known as RNA Polymerases, which synthesize RNA from DNA templates.[4] Discovered via phylogenetic studies of land plants, genes of RNAP IV are thought to have resulted from multistep evolution processes that occurred in RNA Polymerase II phylogenies.[5] Such an evolutionary pathway is supported by the fact that RNAP IV is composed of 12 protein subunits that are either similar or identical to RNA polymerase II, and is specific to plant genomes.[6] Via its synthesis of siRNA, RNAP IV is involved in regulation of heterochromatin formation in a process known as RNA directed DNA Methylation (RdDM).[1][2]