Complex I (nicotinamide adenine dinucleotide (NADH): ubiquinone oxidoreductase) is the largest complex of the mitochondrial oxidative phosphorylation system (OXPHOS) system. Forty-four subunits encoded in nuclear and mitochondrial genomes compose this multiprotein complex， its assembly being a highly complex process involving at least 15 additional nuclear encoded assembly factors. Complex I deficiency is a mitochondrial disorder usually associated with early-onset severe multisystem disorders characterized by highly variable clinical manifestations. Flavin adenine dinucleotide (FAD)-dependent oxidoreductase domain-containing protein 1 (FOXRED1) is a complex I assembly factor. To date， only five patients with mitochondrial complex I deficiency due to mutations in have been characterized. Here， we describe a child with ataxia， epilepsy and psychomotor developmental delay carrying two heterozygous variants， c.920G>A (p.Gly307Glu) and c.733+1G>A. We demonstrate the molecular mechanism supporting the pathogenicity of the FOXRED1 variants， showing a clear deficiency of complex I activity. The reduction in the steady-state level of complex I holoenzyme in patient fibroblasts， confirmed the pathogenicity of the variants and showed the molecular mechanism behind their pathogenicity. A comparison of the clinical presentation of the index case with the previously described cases allowed deepening our knowledge about the clinical variability associated with defects.