PRF1 and neoplasm: NK cells have been shown to identify and attack ovarian CSCs expressing low MHC-class I via direct cytotoxicity, wherein NK cells induce direct cytotoxicity in tumor cells through the perforin/granzyme pathway, where they release perforin to create pores in the tumor cell membrane and granzyme B to trigger caspase-mediated apoptosis (Figure 2) [23,94,95].