Molecular imaging with the cell replication PET tracer 3’-deoxy-3’-18 F-fluorothymidine ([18 F]FLT) has additionally allowed mapping and noninvasive imaging of cell proliferation in secondary lymphoid organs after CTLA4 blockade with tremelimumab in patients with metastatic melanoma [18], as well as the study of the kinetics of lymphocyte subsets in response to dendritic cell vaccination in melanoma patients with lymph node metastases [19]. The gene discussed is CTLA4; the disease is metastatic melanoma.