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. Here, C4A is linked to Sjogren syndrome.