To exclude an influence of very low C4 serum levels on both C4 deposition and MBL pathway activity assays, subanalyses of the association between laboratory variables and the occurrence of infections and major infections were performed after exclusion of SLE patients with C4 serum levels of less than or equal to 0.1 g/l (n = 28). The gene discussed is C4A; the disease is systemic lupus erythematosus.