COX4I1 and Parkinson disease: To further determine whether mitochondrial protein import machinery was impaired in the SN of PD patients, we analyzed the expression levels of two nuclear-encoded mitochondrial proteins, NADH (reduced form of nicotinamide adenine dinucleotide)–ubiquinone oxidoreductase core subunit S3 (NDUFS3) and cytochrome c oxidase subunit IV (COX IV) by immunoblotting.