PRF1 and hemophagocytic syndrome: Familial HLH (FHL) is caused by biallelic defects in PRF1, UNC13D, STX11, or STXBP2 that severely impair granule-mediated cytotoxicity, the process by which perforin/granzyme-containing granules fuse with the plasma membranes of natural killer (NK) cells or cytotoxic T lymphocytes to induce apoptosis in target cells (2).