C4A and arthritic joint disease: Compared with the anti-MDA5-negative group, patients with anti-MDA5-positive DM had shorter disease courses from the disease onset to enrollment (4 ± 7 vs. 11 ± 12 months), higher rates of arthritis (78 vs. 31%) and Gottron sign/papules (78 vs. 38%), lower level of CK (median 67U/L vs. 136U/L), higher level of serum ferritin (median 719 vs. 474 ug/L), and C4 (median 289 vs. 232 mg/L), higher rate of positive CEA (33 vs. 7%, all p < 0.05, Table 1 and Supplementary Table 1).