Dear Editor,
Protein coding mRNAs and non-coding RNAs are extensively modified by chemical modifications,1 including methylation of the N6 position of adenosine (m6A).2 While mRNA internal m6A methylation is mediated by METTL3 and METTL14, which are members of the MT-A70 family,2 N6-methylation of cap-adjacent N6,2'-O-dimethyladenosine (m6Am) is catalyzed by PCIF1.3-6 Non-coding RNAs are also m6Am methylated internally,7 but the corresponding enzyme remains unknown.Here we provide evidence that METTL4 mediates internal m6Am methylation of U2 small nuclear RNA (snRNA).