PRF1 and breast cancer: Many recent studies have reported that NK-exos may enhance tumor-targeting and anti-tumor activity by upregulating cytotoxic proteins (e.g., perforin, granzyme B, FasL, TRAIL, and caspases) in various cancers (e.g., melanoma, neuroblastoma, breast cancer, lung cancer, and pancreatic cancer) [15,16,17,18,19].