Relevant literature has emphasized that the seven genes (AP31B, LYST, PRF1, RAB27A, STX11, STXBP2, UNC13D) known to cause fHLH (classically defined as familial HLH syndromes and hypopigmentation syndromes) [17,71] contribute to the dominant role played by T and NK cells in the development of HLH [72,73,74]. This evidence concerns the gene RAB27A and hemophagocytic syndrome.