Vaccinia capping enzyme is composed of two subunits (D1 and D12). The D1 subunit carries three enzymatic activities (RNA triphosphatase, guanylyltransferase and guanine methyltransferase); all necessary for addition of a complete Cap 0 structure, m7Gppp5’N to 5′ triphosphate RNA (5,6). In vitro transcripts can be capped in less than one hour in the presence of the capping enzyme, reaction buffer, GTP, and the methyl donor, SAM. Capping is nearly 100% efficient and all capped structures are added in the proper orientation, unlike co-transcriptional addition of some cap analogs (7).