COX4I1 and Parkinson disease: Neurons with decreased complex I or IV expression were identified in 7/9 PD cases (complex I: NUDFB8, 8.51–58.14%; NDUFA13, 7.89–46.67%; complex IV: MTCO1, 5.26–37.21%; COXIV, 2.63–58.14%; Fig. 3a, b).