FHL3 is caused by mutations in the UNC13D gene located on chromosome 17q25. UNC13D encodes the protein MUNC 13-4. Found on the surface of lytic granules, MUNC 13-4 is required for priming the lytic granules for docking at the cytotoxic cell membrane [9, 12] . FHL 4 is caused by mutations in STX11 on chromosome 6q24 that encodes the syntaxin 11 protein. Syntaxin-11 is a member of the SNARE protein family and facilitates the fusion of the lytic granule membrane with that of the cytotoxic lymphocyte [1, 23] . FHL5 is caused by mutations in MUNC 18-2 located on chromosome 19p13. MUNC 18-2 encodes the syntaxin binding protein 2. It is a partner of syntaxin 11 and is required for SNARE complex-mediated fusion of the lytic granule with the cytolytic cell membrane [9, 22] .