C3 and systemic lupus erythematosus: The results showed that the expression of circPTPN22 in patients with SLE and alopecia, arthritis, lupus nephritis, anti-dsDNA ( +), and C3 or C4 deficiency was significantly lower than that in patients without these symptoms (all P < 0.05), but there was no significant difference between patients with skin involvement, leukopenia, anti-Sm ( +), and patients without these symptoms (Fig. 1D–K).