RNGTT (RNA guanylyltransferase and 5'-phosphatase) is a 597 amino acid protein that localizes to the nucleus and exists as four alternatively spliced isoforms. Expressed in kidney, lung, heart, brain, liver, testis, skin and muscle, RNGTT acts as a bifunctional mRNA-capping enzyme that exhibits RNA 5'-triphosphatase activity at its N-terminus and mRNA guanylyltransferase activity at its C-terminus. Via its catalytic activity, RNGTT facilitates the first two steps of mRNA cap formation, naming the removal of a γ-phosphate from the end of nascent mRNA and the subsequent transfer of the phospho residue to the 5’-diphosphate terminus. The gene encoding RNGTT maps to human chromosome 6, which contains 170 million base pairs and comprises nearly 6% of the human genome.