ETHE1 and Menkes disease: Ethylmalonic encephalopathy (EE) is a severe monogenic disorder caused by mutations of the nuclear gene ETHE1, encoding a ubiquitous mitochondrial protein sulfur dioxygenase (SDO) (note: EE is not classified as MD) [131], involved in the detoxification of H2S [132, 133] showed that AAV2/8-mediated, ETHE1-gene transfer to the liver of a genetically, metabolically, and clinically faithful EE mouse model resulted in the full restoration of SDO activity, correction of plasma thiosulfate, a biomarker reflecting the accumulation of H2S, and spectacular clinical improvement [133].