PRF1 and hemophagocytic syndrome: Primary HLH is associated with several inheritable gene defects, most prominently variants in PRF1 (perforin 1), UNC13D, STX11 (Syntaxin 11), MUNC18-2 (also called STXBP2, Syntaxin Binding Protein 2) or RAB27A (13, 15, 16).