For the functional analysis of protease inhibitor X (PI-X) which was preferentially expressed in CD44-positive fraction, we evaluated the effect of PI-X knockdown by siRNA or PI-X overexpression in CRC cell lines on the sensitivity to CTL lysis in vitro and the effect of PI-X overexpression in murine CRC cells on the therapeutic efficacy of anti PD- 1 therapies in vivo. The gene discussed is RPL17; the disease is colorectal carcinoma.