Given the recently described association between serum FGF-21 elevation and muscle COX defects in inherited mitochondrial disorders [18], and the recent observation of significant COX defects in long-term anti-retroviral treated HIV-infected patients [8], we speculated that muscle mitochondrial dysfunction might also drive the FGF-21 elevation in anti-retroviral treated HIV infection. The gene discussed is FGF21; the disease is HIV infectious disease.