Several mouse studies and early clinical trials have already indicated the value of noninvasive imaging of CD4+ cells in rheumatoid arthritis (20), colitis (21), allogeneic stem cell transplantation (44), organ transplant rejection (45), acquired immunodeficiency disease (10), and in the context of cancer immunotherapies (46), using radiolabeled full-length antibodies or fragments thereof. The gene discussed is CD4; the disease is rheumatoid arthritis.