UQCC2 (ubiquinol-cytochrome c reductase complex assembly factor 2) is essential for assembling mitochondrial respiratory chain complex III (cytochrome b-c1 complex) 1. The protein functions early in complex III assembly, upstream of UQCC3, and is required for proper cytochrome b stability and integration into the holocomplex 2. UQCC2 regulates oxidative phosphorylation, directly impacting ATP production and oxygen consumption 3, processes critical for skeletal muscle differentiation and pancreatic beta-cell insulin secretion. Additionally, miR-663 directly regulates UQCC2 expression to maintain respiratory supercomplex stability 3, and UQCC2 cooperates with SMIM4 and UQCC1 in early complex III biogenesis linked to the mitochondrial translation machinery 4. Loss-of-function mutations in UQCC2 cause mitochondrial complex III deficiency, a severe multisystem disorder presenting with lactic acidosis, hypoglycemia, developmental delays, and combined respiratory chain deficiency affecting complexes I and III 1. UQCC2 variants are also associated with ADHD susceptibility in genome-wide studies 5, suggesting broader neurodevelopmental roles beyond classical mitochondrial disease.