NDUFB4 and Alzheimer disease: The iTRAQ-quantitative brain proteomic data revealed down-regulation of subunits NDUFA2, NDUFA3, NDUFA7, NDUFA13, NDUFB3, NDUFB4, NDUFB6, NDUFB11, NDUFS1, NDUFS3, NDUFS4 and NDUFS7 of complex I in the late-onset AD patients.