Levels of IgM against IGKC2−19 in RA patients were greatly higher than the levels in OA patients by 2.24-fold (p < 0.0001) and HCs by 2.48-fold (p < 0.0001) (Fig. 3k, left panel), and AUC values were 0.83 (with 81.7% sensitivity and 60.0% specificity) for RA detection and 0.56 (with 57.1% sensitivity and 50.0% specificity) for detecting OA (Table 2). This evidence concerns the gene CD40LG and rheumatoid arthritis.