RNMTL1 (also known as HC90, MRM3 and RMTL1) catalyzes the methylation of 2'-O-ribose residues G1370 to GmG residue of the mitochondrial 16S rRNA. RNMTL1 is a member of the SPOUT (SpoU-TrmD) methyltransferase (MTase) superfamily, a large class of S-adenosyl-L-methionine (AdoMet or SAM)-dependent RNA MTases which are structurally characterized by a deep trefoil knot.