THG may refer to:
TRNAHis guanylyltransferase (EC 2.7.7.79, histidine tRNA guanylyltransferase, Thg1p, Thg1) is an enzyme with system name p-tRNAHis:GTP guanylyltransferase (ATP-hydrolysing). This enzyme catalyses the following chemical reaction
The enzyme requires a divalent cation for activity.