In a related disease, juvenile idiopathic arthritis, CD27 + IgD‐ and CD27 – IgD‐ B‐cells accumulate in the joints of patients, and express somatically hypermutated and class‐switched BCRs.110 Indeed, these cells displayed activated phenotypes, expressing co‐stimulatory molecules CD80/CD86 and were able to activate allogeneic T‐cells more potently than their peripheral blood B‐cell counterparts, consistent with a role in driving disease. The gene discussed is CD27; the disease is juvenile idiopathic arthritis.