Two recent single-center studies (11, 12) in the U.S. and China have characterized the diagnostic sensitivity and specificity of complement protein 4 derived ligand (C4d) bound to CD3+ T Cells (TC4d) in combination with TIgG and TIgM, compared to conventional autoantibodies (i.e., anti-dsDNA, anti-Smith) and serum complement proteins (i.e., C3 and C4) traditionally used in the diagnosis of SLE. The gene discussed is C4A; the disease is systemic lupus erythematosus.