PRF1 and hemophagocytic syndrome: On the other hand, the familial or primary form of hemophagocytic lymphohistiocytosis (fHLH/pHLH) occurs due to autosomal recessive defects in genes (PRF1; UNC13D; STX11; STXBP2) encoding proteins involved in cytotoxic granule exocytosis of NK-induced apoptosis [4,5,6].