COQ9 is a membrane-associated protein essential for coenzyme Q (ubiquinone) biosynthesis, a critical lipid-soluble electron transporter required for aerobic respiration 1. COQ9 functions as a lipid-binding protein that warps the mitochondrial inner membrane to access and present aromatic isoprene intermediates to COQ7, facilitating the COQ7-mediated hydroxylase step in CoQ synthesis 2. Mechanistically, COQ9 and COQ7 form multimeric complexes where two COQ7:COQ9 heterodimers assemble into curved tetramers that deform the membrane, creating a pathway for CoQ intermediates to translocate from the lipid bilayer to protein-binding sites 2. COQ9 stabilizes the mitochondrial CoQ-synthome complex, enhancing overall CoQ biosynthesis efficiency 3. Pathogenic COQ9 mutations cause primary coenzyme Q10 deficiency, a mitochondrial disorder presenting with variable phenotypes ranging from severe neonatal-onset multisystemic disease to childhood-onset hereditary spastic paraplegia 34. Clinically, reduced COQ9 function impairs cellular respiration and antioxidant defense, particularly affecting high-energy tissues like brain and muscle 5. Importantly, CoQ10 headgroup intermediates can therapeutically restore CoQ synthesis in COQ9-related deficiencies, offering potential treatment avenues for these patients 5.