RTN4IP1 and inborn mitochondrial metabolism disorder: Recessively inherited, pathogenic variants in the RTN4IP1 gene were first identified in several families with isolated optic atrophy (Angebault et al, 2015), leading to the characterisation of larger patient cohorts with variable neurological phenotypes, thus expanding the clinical phenotype of RTN4IP1-related mitochondrial disease (Charif et al, 2018).