For the third patient, we report a novel pathogenic variant m.3955G>A p.(Ala217Thr) in MT-ND1 as a new cause of Leigh syndrome in a boy who presented at 2 months with an early infantile epileptic encephalopathy, lactic acidosis, and brain MRI demonstrating bilateral symmetrical signal abnormality involving the caudate nuclei, putamina, midbrain, medulla, upper cervical cord, and perirolandic cortex. The gene discussed is MT-ND1; the disease is lactic acidosis.