Here, we present a ~2.5 Å structure of the methyltransferase core of human METTL3-METTL14 in complex with the reaction product mimic, N6-methyladenosine monophosphate (m6A), representing a state post-catalysis but before the release of m6A. m6A occupies an evolutionarily conserved RNA-binding pocket ~16 Å away from the SAM pocket that also frequently mutates in cancer. This evidence concerns the gene METTL3 and cancer.