C4A and systemic lupus erythematosus: al. [85] demonstrated that polymorphisms of the C4 protein of the complement system, which is linked to classical HLA genes, is a strong predictor of sex-biased vulnerability to three human illnesses (the autoimmune diseases systemic lupus erythematosus (SLE), Sjögren’s syndrome, and schizophrenia), highlighting the need to explore other genes within the MHC in association studies.