In a Caucasian boy with HSD10MD, Falk et al. (2016) identified a hemizygous missense mutation in the HSD17B10 gene (K212E; ). No parental DNA or DNA from reportedly affected maternal uncles was available for study. In vitro functional expression assays showed that the mutation resulted in decreased dehydrogenase activity. However, more significantly, the mutation disrupted TRMT10C ( 615423 )-associated methyltransferase activity and destabilized the RNase P holoenzyme, resulting in impaired mitochondrial tRNA processing and maturation and impaired mitochondrial protein synthesis. The findings suggested that the major pathogenic mechanism resulting from HSD17B10 mutations is the adverse effect on mitochondrial function.