To do so, we discuss the role of PD-1 and its circadian control in different immune cell types and pathways, including tumor-associated macrophages (TAMs), myeloid-derived suppressor cells (MDSCs), macrophage migration inhibitory factor (MIF) and its cognate receptor CD74, circulating tumor cells (CTCs), and T cells/Tregs. The gene discussed is MIF; the disease is neoplasm.