UQCRH (ubiquinol-cytochrome c reductase hinge protein) is a structural component of mitochondrial respiratory complex III, functioning as the hinge protein that facilitates electron transfer between cytochrome c1 and cytochrome c during oxidative phosphorylation 1. This protein is essential for the Q-cycle mechanism, which couples electron transfer to proton translocation across the inner mitochondrial membrane, generating the electrochemical gradient necessary for ATP synthesis. Loss-of-function mutations in UQCRH cause mitochondrial complex III deficiency, characterized by impaired respiratory chain activity and severe metabolic dysfunction. Patients with homozygous UQCRH deletions present with recurrent lactic acidosis, hyperammonaemia, and encephalopathy 1. Animal models show cardiac contractile dysfunction, bioenergetic impairment, and failure to thrive following weaning 2. Conversely, UQCRH downregulation through DNA hypermethylation promotes cancer progression. In renal cell carcinoma, UQCRH hypermethylation silences expression and exacerbates the Warburg effect, promoting tumor growth 3. UQCRH functions as a tumor suppressor by facilitating mitochondrial-mediated apoptosis 4. In hepatocellular carcinoma, UQCRH overexpression paradoxically associates with poor prognosis and shorter survival 5, and elevated UQCRH correlates with venous thromboembolism in cirrhotic patients 6. These dichotomous cancer associations suggest tissue-specific or context-dependent roles in tumorigenesis.