PRF1 and hemophagocytic syndrome: The diagnosis of HLH is made on the basis of the presence of mutations associated with primary HLH (such as mutations in PRF1, UNC13D, or STX11) and/or clinical and laboratory criteria, such as fever, cytopenias, hypertriglyceridaemia, hypofibrinogenaemia, elevated serum levels of ferritin and liver enzymes, haemophagocytosis, low or absent NK cell activity, and/or elevated soluble IL-2 receptor levels113.