TRMT10C in TRMT10C:HSD17B10 (TRMT10C:SDR5C1), a subcomplex of the mitochondrial RNase P complex, methylates the 1 position of guanosine-9 in mitochondrial tRNAs (Vilardo et al. 2012). 5 of 22 mitochondrial tRNAs have a G9 residue. Methylation of G9 appears to be important for correct folding of tRNA. Mutations in the 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).