The mismatch repair (MMR) pathway is used by normal proliferating cells to repair mutations that may occur during DNA replication. Loss of function of any of the four MMR proteins, MLH1, PMS2, MSH2, or MSH6, results in MMR deficiency (dMMR) and can lead to an increased mutation rate, promotion of tumorigenesis, and generation of neoantigens. dMMR tumors may be more responsive to immunotherapies than tumors with functioning MMR pathways due to the increased presence of neoantigens and immune cell recruitment.