Responsible for methylating the 5'-cap structure of mRNAs. a 5'-end (5'-triphosphoguanosine)-(ribonucleoside) in mRNA + S-adenosyl-L-methionine = a 5'-end (N(7)-methyl 5'-triphosphoguanosine)-ribonucleoside in mRNA + S-adenosyl-L-homocysteine Belongs to the class I-like SAM-binding methyltransferase superfamily. mRNA cap 0 methyltransferase family.