C4A and Alzheimer disease: One of these networks, M01, was specific to AD cases with the APOE ε2/ε3 genotype and contained multiple genes in the classical complement pathway including C4A, C4B, and CR1. Expression of C4A, C4B, and HSPA2 was significantly associated with amyloid plaque and neurofibrillary tangle density, as well as with the ratio of phosphorylated tau at protein position 231 to total Tau (pTau231/tTau).