Bi-allelic mutations in UNC13D (encoding Munc13-4, accounting for about one third of FHL cases), STX11 (encoding syntaxin 11, about 5% of FHL cases), and STXBP2 (encoding syntaxin-binding protein 2, also known as Munc18-2, about 10–15% of FHL cases) result into the occurrence of HLH in FHL types 3, 4, and 5, respectively (13–16). The gene discussed is UNC13D; the disease is hemophagocytic syndrome.