Next, we tried to install two mutations15,16 at GC context in MT-ND4 and MT-ND6 that have been linked to human diseases including Leber hereditary optic neuropathy (LHON) and Leigh syndrome (Fig. 3a), which are both devastating genetic diseases with no effective therapy. The gene discussed is MT-ND6; the disease is Leber hereditary optic neuropathy.