Mutations occur in genes such as PRF1, which encodes perforin [5,16,17], UNC13D, which encodes Munc13-4 and is critical for the release of cytolytic vesicles [18,19], STX11 (Syntaxin-11) [20] and STXP2 (Munc18-2), which are involved in cytolysis [21,22], and RAB27A (Rab27a) [23,24], which are responsible for familial HLH forms [7]. Here, PRF1 is linked to hemophagocytic syndrome.