NDUFAF3 is an essential mitochondrial assembly factor required for complex I (NADH:ubiquinone oxidoreductase) biogenesis. It functions as a structural component facilitating the assembly of complex I subunits, interacting directly with NDUFAF4 and other assembly proteins 1. NDUFAF3 coordinates the sequential assembly of functional modules, including the membrane arm and Q-module/P-module components, ensuring proper complex I maturation and respiratory supercomplex formation 2. Pathogenic NDUFAF3 variants cause mitochondrial complex I deficiency manifesting as autosomal recessive mitochondrial disease 3. Clinical presentations range from severe neonatal presentations with fatal outcomes and lactic acidosis 1 to more variable phenotypes including Leigh syndrome, cavitating leukoencephalopathy 4, progressive optic atrophy 5, and dystonia with basal ganglia abnormalities 2. Biochemically, NDUFAF3 deficiency results in reduced complex I activity, impaired assembly of early sub-complexes, and accumulation of intermediate assembly forms. Disease severity correlates with the degree of complex I dysfunction; assembly factor variants generally produce more variable phenotypes compared to core subunit defects 5. These findings establish NDUFAF3 as a critical determinant of mitochondrial respiratory function with broad neurological disease implications.