PRF1 and hemophagocytic syndrome: Additionally, there is evidence that some cases [14] of HLH may be attributable to a combination of a single heterozygous pathogenic variant in LYST plus another heterozygous pathogenic variant in another gene involved in the cytotoxic lymphocyte degranulation pathway (PRF1, UNC13D, STX11, STXBP2, AP3B1, or RAB27A) (i.e., digenic inheritance) [15, 16].