C4A and systemic lupus erythematosus: Genetic variants resulting in non-expression of C4A and C4B genes (so-called “null” alleles, C4A*Q0 and C4B*Q0) are common in healthy European populations and have shown association with a number of diseases, most notably the autoimmune disease, systemic lupus erythematosus (SLE/lupus; MIM: 152700) [3], [4], [5], [6].