PRF1 and gastric cancer: Cytolytic activity has been previously shown to correlate with oncogenic viruses such as with HPV in cervical cancer, with EBV infection in stomach cancer, and with HBV and HCV infection in liver cancer24, and our results of different PRF1 genotypes in the order of group A, B, and C may suggest the possibility of different amount of tumor infiltrating lymphocytes due to different availability of HPV viral antigen.