Monocytes and dendritic cells regulate adaptive and innate immunity. This study uncovers an association between mutations in the gene encoding interferon regulatory factor 8 and deficiency of dendritic cells and monocytes in the context of disseminated bacille Calmette–Guérin disease. The discovery of human primary immunodeficiencies that affect the development of granulocytes, B cells, and T cells has been instrumental in defining the contribution of these cell types to protective immunity. 1 , 2 Monocytes, macrophages, and dendritic cells — all mononuclear phagocytes — have essential functions in both innate and acquired immunity. These cells initially recognize and engulf invading microbes, produce proinflammatory cytokines (e.g., interleukin-12), and process antigens for presentation to naive T cells, which consequently secrete various lymphokines (e.g., interferon-γ). 3 , 4 On activation by cytokines secreted by T cells, mononuclear phagocytes destroy ingested microorganisms. There are no known genetic causes . . .