TRMT10C of TRMT10C:HSD17B10 (TRMT10C:SDR5C1), a subcomplex of the mitochondrial RNase P complex, methylates the 1 position of adenosine-9 in mitochondrial tRNAs (Vilardo et al. 2012). 14 of 22 mitochondrial tRNAs have an A9 residue. Methylation of A9 appears to be important for correct folding of tRNA (Helm et al. 1998). Mutations in the HSD17B10 (SDR5C1) dehydrogenase subunit of RNase P impair dehydrogenation, tRNA methylation, and tRNA processing, causing HSD10 disease, which is characterized by progressive neurodegeneration and cardiomyopathy (Vilardo and Rossmanith 2015).