Enzyme replacement by allogenic hematopoietic stem cell transplantation (HSCT) has shown promising initial results with MNGIE due to mutations in the TYMP gene encoding thymidine phosphorylase, another enzyme involved in nucleoside metabolism (Nishino et al, 1999; Hirano et al, 2006; Garone et al, 2011) but limited to disorders with toxic metabolites that can be eliminated by circulating cells. This evidence concerns the gene TYMP and mitochondrial neurogastrointestinal encephalomyopathy.