PRF1 and hemophagocytic syndrome: compared the NK cell cytotoxicity assay, NK cell perforin, and CD107a expression in a large HLH cohort and discovered that perforin and CD107a expression were more sensitive and not less specific than NK cell cytotoxicity tests for screening genetic HLH, with reported sensitivities of 59.5%, 96.6%, and 93.8% and specificities of 72.0%, 99.5%, and 73%, respectively (68).