We previously identified a series of patients, including one with isolated CI deficiency, presenting with variable neurological phenotypes ranging from isolated optic atrophy to severe early-onset encephalopathy due to pathogenic, bi-allelic, variants in the mitochondrial Reticulon-4-interacting protein 1 (RTN4IP1) gene (Charif et al, 2018). Here, RTN4IP1 is linked to optic atrophy.