Thymidine phosphorylase is not physiologically expressed in skeletal muscle, but the muscle from patients with MNGIE shows alterations in mtDNA, COX–deficient and ragged red fibers and respiratory chain enzymatic defects (Yoshimura et al., 1990; Hirano et al., 1994; Nishino et al., 1999). The gene discussed is TYMP; the disease is mitochondrial neurogastrointestinal encephalomyopathy.