FSIP1 and HSD10 mitochondrial disease: HSD10 mitochondrial disease (HSD10MD), originally described as a deficiency of 2‐methyl‐3‐hydroxybutyryl‐CoA dehydrogenase (MHBD), is a rare X‐linked disorder of a moonlighting protein encoded by the HSD17B10. The diagnosis is usually first suspected on finding elevated isoleucine degradation metabolites in urine, reflecting decreased MHBD activity.