Extensive analyses of gene copy number variations for total C4, C4A, and C4B, plasma protein levels of total C4, C4A, C4B, C3, factor H and MBL, and antiphospholipid antibodies revealed distinct patterns of diversity that can be relevant and effective quantitative biomarkers for thrombosis, SLE and recurrent pregnancy loss. The gene discussed is C4B; the disease is systemic lupus erythematosus.