PRF1 and neoplasm: The mechanism of CIK cells killing tumors is as follows: first, CIK cells recognize and bind tumor antigens on tumor cells through their antigen receptors, and directly kill tumor cells through cell lysis and cytotoxic molecules such as granzyme and perforin.[12,13] Second, CIK cells indirectly kill tumor cells by secreting several cytokines such as interferons (IFNs) and tumor necrosis factors (TNFs).[14,15]