PRF1 and hemophagocytic syndrome: Accumulation of reported cases and investigation of consanguineous families allowed to define that the familial form of HLH is indeed a congenital immune deficiency; to date, four subtypes are defined by mutations in the following genes: PRF1 in familial hemophagocytic lymphohistiocytosis type 2 (FHL2), UNC13D in familial hemophagocytic lymphohistiocytosis type 3 (FHL3), STX11 in familial hemophagocytic lymphohistiocytosis type 4 (FHL4), and STXBP2 in familial hemophagocytic lymphohistiocytosis type 5 (FHL5).