PRF1 and hemophagocytic syndrome: At the transcriptional level in T cells and NK cells of EBV-HLH patients, there was no observed downregulation of cytotoxic genes, such as PRF1 and GZMB. This finding suggests that the decreased CTL killing ability in EBV-HLH is not due to a deficiency in cytotoxic-related functions, which is a notable difference from familial HLH.