Congenital defects affecting the perforin-mediated lymphocyte cytotoxicity, such as autosomal recessive mutations in PRF1, UNC13D, STX11, and STXBP2, represent the most common causes of primary HLH, termed familial HLH (FHL) type 2, 3, 4 and 5, respectively [1, 4]. The gene discussed is PRF1; the disease is hemophagocytic syndrome.