There are presently five types of familial HLH (FHL) [9] , all of which impair the lytic activity of cytotoxic lymphocytes. Specifically, cytotoxic lymphocytes mediate contact-dependent elimination of cells perceived as dangerous by secreting preformed destructive molecules contained within specialized organelles termed lytic granules [2] . In order to mediate cytotoxicity, lytic granules must contain appropriate effector molecules and be localized to the contact site with the cell targeted for destruction. Once localized to that intercellular interface, the lytic granules dock at the cell membrane. The membrane of the granule and the cytotoxic lymphocyte are fused, allowing the release of the lytic effector molecules onto the targeted cell.