C3 levels (0.4174 ± 0.04605 vs. 0.5646 ± 0.06724, p = .0072) and C4 levels (0.08030 ± 0.01303 vs. 0.1255 ± 0.01930, p = .0321) were significantly lower in patients with MAS as an initial manifestation of SLE compared with patients with MAS during the course of lupus. This evidence concerns the gene C3 and macrophage activation syndrome.