C4A and Alzheimer disease: As for AD, dedicated GWAS studies delivered strong indication that complement is an unexpected but major player in schizophrenia: the extended major histocompatibility complex (xMHC) represents a region of strong association with schizophrenia, and fine mapping of this region then identified copy number variants of the C4 genes (C4A and C4B) as the most significant risk factor (211–214).