MLH1 and mismatch repair cancer syndrome 1: More prominent in this population is the prevalence of mismatch repair deficiency (MMR-D) or microsatellite instability-high (MSI-high) disease, in up to 5% of metastatic CRC (mCRC).3 MMR-D is characterized by the loss of expression or function of any of the MMR genes including, but not limited to, MLH1, PMS2, MSH2, or MSH6.