In patients in whom the diagnosis of MATD was based on metabolite data only (without rather labile 2-methylacetoacetic acid), but not confirmed on enzyme and/or mutation level, it needs to be considered that patients were possibly not affected by MATD but by HSD10 mitochondrial disease (HSD10MD; OMIM 300438). The gene discussed is FSIP1; the disease is inborn mitochondrial metabolism disorder.