Familial hemophagocytic lymphohistiocytosis-5 with or without microvillus inclusion disease (FHL5) is an autosomal recessive hyperinflammatory disorder characterized clinically by fever, hepatosplenomegaly, pancytopenia, coagulation abnormalities, and other laboratory findings. Some patients have neurologic symptoms due to inflammatory CNS disease. There is uncontrolled and ineffective proliferation and activation of T lymphocytes, NK cells, and macrophages that infiltrate multiple organs, including liver, spleen, lymph nodes, and the CNS. The phenotype is variable: some patients may present in early infancy with severe diarrhea, prior to the onset of typical FHL features, whereas others present later in childhood and have a more protracted course without diarrhea. The early-onset diarrhea is due to enteropathy reminiscent of microvillus inclusion disease (see MVID, 251850). The enteropathy, which often necessitates parenteral feeding, may be the most life-threatening issue even after hematopoietic stem cell transplantation (HSCT). More variable features include sensorineural hearing loss and hypogammaglobulinemia. Treatment with immunosuppressive drugs and chemotherapy can ameliorate signs and symptoms of FHL in some patients, but the only curative therapy for FHL is HSCT. HSCT is not curative for enteropathy associated with the disorder, despite hematologic and immunologic reconstitution (summary by Meeths et al., 2010; Pagel et al., 2012; Stepensky et al., 2013). For a phenotypic description and a discussion of genetic heterogeneity of familial hemophagocytic lymphohistiocytosis (FHL, HLH), see 267700.