STXBP2 and hemophagocytic syndrome: Five different forms of FHL have been described based on defects in different genetic material and genes, including chromosome arm 9q mutations (FHL1), PRF1 (FHL2), UNC13D (MUNC13-4) (FHL3), STX11 (FHL4), and STXBP2 (MUNC18-2) (FHL5) [11–17].