AP3B1 and Chédiak-Higashi syndrome: Primary HLH can be caused by mutations in a number of genes which affect cytotoxic lymphocyte granule-mediated cytotoxicity including PRF1, UNC13D, STX11, STXBP2, RAB27A (Griscelli Syndrome), AP3B1 (Hermansky-Pudlak syndrome type 2), and LYST (Chediak-Higashi Syndrome).