CD28 and rheumatoid arthritis: Cao J et al demonstrated higher serum levels of sCTLA‐4 and sCD28 in RA patients than in healthy controls, where serum sCTLA‐4 concentration exhibited a positive and significant correlation with DAS28 score in all RA patients; thus, they proposed that serum levels of sCTLA‐4 could serve as a new marker of RA disease activity; however, they did not evaluate the surface expression of CTLA‐4 and CD28.16