STXBP2 and Griscelli syndrome type 2: Since 1999, several genes have been associated with FHL: PRF1 (OMIM *170280), UNC13-D (OMIM *608897), STX11 (OMIM *605014), and STXBP2 (OMIM *601717), all encoding proteins that play a key role in lymphocyte-mediated cytotoxicity.14-17 In addition, biallelic mutations in RAB27A (OMIM *603868)18 give rise to a form of FHL known as Griscelli syndrome type 2 (GS2).