STXBP2 and familial hemophagocytic lymphohistiocytosis type 1: Mutations in the HPLH1 gene are responsible for familial hemophagocytic lymphohistiocytosis type 1 (FLH-1), a mutation in the PRF1 gene causes FLH-2, mutations in the UNC13D gene cause FHL-3, mutations in the STX11 gene cause FHL-4, and a mutation in the STXBP2 (UNC18B) gene causes FHL-5 [69].