PRF1 and cancer: Although the effect of NK-92MI cell-based immunotherapy on ATC cells has been shown in vitro, the tumor microenvironment could decrease the antitumor effect of NK cells by reducing the release of perforin/granzymes from NK cells and inhibiting Fas/FasL interaction (34, 35); therefore, in vivo studies might be more important than in vitro experiments to clearly demonstrate the therapeutic effect of NK cells on cancer.