C4A and schizophrenia: While C4B null alleles that decrease the expression of C4 have been associated with ASD [16,17,18], alleles at C4A that increase the expression of C4 have been associated with schizophrenia, and, mechanistically, it has been proposed that augmented C4 is involved in the exacerbated synaptic pruning and decreased synapse number in schizophrenic patients [8].