C4A and systemic lupus erythematosus: On the contrary, the rs6920220 SNP showed correlations only with SLE clinical phenotypes: the variant allele seems to confer a higher risk to develop pericarditis (P = 0.007, OR = 2.25), pleurisy (P = 0.033, OR = 2.12), and nephritis (P = 0.0004, OR = 2.48) but also confers a protective effect with respect to the reduction of C4 (P = 0.012, OR = 0.50) and C3 (P = 0.026, OR = 0.55) levels and with respect to the development of thrombocytopenia (P = 0.017, OR = 0.36) (Table 3).