PRF1 and hemophagocytic syndrome: Soon thereafter, mutations in other genes (STX11, STXBP2, UNC13D) with related functions in the cytotoxic response were implicated in HLH, resulting in a nomenclature of familial HLH (FHL), where ‘familial’ implied inherited, and a corresponding number implied a specific genetic defect in granule mediated cytotoxicity (e.g., FHL2 is HLH due to a perforin defect).