Pooled SLE patient sera after 52 weeks of atacicept treatment were sequentially immunoprecipitated first with anti-BAFF mAb2.81 and second with anti-APRIL mAb Heaty-1, or vice versa, and then analyzed by Western blot anti-APRIL and anti-BAFF. This evidence concerns the gene TNFSF13B and systemic lupus erythematosus.