(A) Cartoon diagrams of METTL14 in structures determined in this study (left, light pink) and by Wang et al. (2016b) (right, colored dark purple). The structures are shown in the same orientations. The principal differences concern the presence or absence of an ordered polypeptide linker connecting the N-terminal helical motif of METTL14 (HM) and the methyltransferase domain (disordered linker indicated with a dashed line). Furthermore, due to a different C-terminal boundary, the structure reported in this study lacks a C-terminal helix in METTL14. Instead, the C-terminal region of METTL14 forms a disulfide bridge between residues Cys338METTL14 and Cys388METTL14. (B) Superposition of the METTL3 actives sites in METTL3-METTL14 structures determined in this study (left, light pink) and by Wang et al. (2016b) (right, colored dark purple).