PRF1 and isolated hyperferritinemia: Although sIL-2R and NK-cell function were unobtainable because of coagulopathy and a very short clinical window, the combination of fever, bicytopenia, hyperferritinemia, hypofibrinogenemia, rapidly escalating organ dysfunction, and postmortem confirmation of biallelic PRF1 loss collectively supported a diagnosis of familial HLH (FHL2) in the neonatal period.