Our study shows that MTS AAV transfer of wild-type human ND4 is nonpathogenic to host mitochondria, and supports our previous studies that indicate an MTS AAV-ND4 can be used to rescue visual loss and optic neuropathy induced by the mutant ND4 allele responsible for most cases of LHON [14]. The gene discussed is MT-ND4; the disease is optic nerve disorder.