In enzymology, a mRNA (guanine-N7-)-methyltransferase also known as mRNA cap guanine-N7 methyltransferase is an enzyme that catalyzes the chemical reactionS-adenosyl-L-methionine + G(5')pppR-RNA S-adenosyl-L-homocysteine + m7G(5')pppR-RNA (mRNA containing an N7-methylguanine cap)