Function of HLH-associated proteins in NK cell cytotoxicity. LYST (function not fully understood) and AP-3 participate in protein trafficking and sorting from the Golgi apparatus to secretory lysosomes and cytolytic vesicles. Rab27a aids in vesicle trafficking and docking at the cell membrane. Munc13-4, Munc18-2, and Syntaxin 11 enable release of granule contents by priming (Munc13-4) and facilitating fusion (Munc18-2, Syntaxin 11) of the vesicle membrane with the plasma membrane. Perforin then forms a porelike structure on the membrane of the target cell, through which granzymes pass to mediate target cell death. CD107a is found within the membrane of cytolytic vesicles, and its expression increases on the surface of the NK cell as a consequence of cytolytic granule release. Flow cytometric assays to measure perforin and CD107a expression use fluorescently conjugated antibodies to quantify the amount of perforin within cytolytic vesicles (here shown by antibodies marked with a red signal) and the levels of CD107a (antibodies with green signal) on the surface of the NK cell after stimulation. Expected results for perforin expression, CD107a upregulation, and NK cell cytotoxicity in patients with primary HLH or other HLH-associated disorders are as shown. Figure prepared with assistance from Joshua Stokes from Biomedical Communications at St Jude Children’s Research Hospital and Patrick Lane from ScEYEnce Studios.