MSH2 and neoplasm: In addition, when a PD-1 inhibitor was used in the MMR-d tumor cell environment, the CD8+ T cell activity was promoted, compared to that in the mismatch-proficient (MMR-p) tumor cell environment, in which all MMR proteins, including MLH1, MSH2, MSH6, and PMS2, were expressed and tumor cell apoptosis was further increased [24].