In enzymology, a mRNA (guanine-N7-)-methyltransferase also known as mRNA cap guanine-N7 methyltransferase is an enzyme that catalyzes the chemical reaction S-adenosyl-L-methionine + G(5')pppR-RNA S-adenosyl-L-homocysteine + m7G(5')pppR-RNA (mRNA containing an N7-methylguanine cap) Thus, the two substrates of this enzyme are S-adenosyl methionine and , whereas its two products are S-adenosylhomocysteine and . This enzyme belongs to the family of transferases, specifically those transferring one-carbon group methyltransferases.