FTO demethylates 6meA, which is the most common modified nucleoside found in mRNA,144 with 50-fold greater affinity than 3meU,142 is found largely in ribosomal RNA.145 However, because the vast majority of total RNA is rRNA, there are in absolute terms actually a hundred fold more 3meU than 6meA in any given cell. The question of whether one or both of these modified bases are the endogenous substrate/s for FTO is still not entirely clear.
