PD-1 protein can be detected in normal murine thymus and spleen T cells at low levels (8), but is strongly induced on thymocytes and T cells in the spleen and lymph nodes after stimulation with an anti-CD3 mAb in vitro (9) and increased on T cells in the spleen and liver after tumor cell injection in vivo (10). The gene discussed is PDCD1; the disease is neoplasm.