Studies indicate that CD20+ T cells may have a protective function in cancer (5–7) and viral infections (8–10) and a pathogenic role in CD20+ T-cell malignancies (11, 12) and various autoimmune disorders including multiple sclerosis (1, 13–17), psoriasis (18), and rheumatoid arthritis (19, 20). Here, MS4A1 is linked to autoimmune disease.