C4A and schizophrenia: For example, the genes coding for complements 4A (C4A) and 4B (C4B) are distinguished by the presence or absence of a human endogenous retroviral (HERV) insertion (intron 9) that expands the C4 gene from 14kb to 21kb without changing the C4 protein sequence; the C4-HERV sequence could increase the ratio of C4A to C4B expression, which was found to be strongly associated with schizophrenia (30).