CD40 and systemic sclerosis: The selected four proteins (calcitonin, fibronectin, CD40, and SOST) were further verified by ELISA tests, among which the serum calcitonin level was confirmed to be upregulated in both SSc and SSc-PAH patients, and the serum SOST level was confirmed to be equally upregulated in each pulmonary complication subgroups of patients with SSc.