GDF6 and rheumatoid arthritis: Compared with previously reported serum N-glycan biomarkers, such as the ratio of G0/G1 (Fig. 7c–e)3–5, 10, 35, the newly identified N-glycan biomarkers, especially SGm1 and SGm2, exhibited higher potential for the classification of RF/ACPA-negative RA patients, (Table 2).