tRNA-guanine transglycosylase (TGT; EC 2.4.2.29) synthesizes queuosine (Q), which is found in tRNAs that recognize NAU and NAC codons, encoding tyr, asn, asp, and his. Prokaryotic TGT is a single protein of 43 kD. In contrast, mammalian TGT appears to be a heterodimer consisting of a 60-kD subunit (USP14; MIM 607274) and a 43-kD catalytic subunit (QTRT1) (Deshpande and Katze, 2001 [PubMed 11255023]).