pMMR means that the DNA mismatch repair gene (i.e., MMR gene) is expressed normally and does not exhibit mutations. MMR gene mutations mainly refer to insertion or deletion mutations at repeated sequence stretches of genes called microsatellite instability (MSI).The MSI pathway can cause diseases such as colorectal cancer and endometrial cancer with some probability. Normal expression of the MMR gene, pMMR, is characterized by low-frequency microsatellite instability (MSI-L) and microsatellite stability (MS-S). Defective MMR gene expression, dMMR, is characterized by high-frequency microsatellite instability (MSI-H). Current methods for examining MMR gene expression include immunohistochemistry, PCR, and second-generation sequencing. Immunohistochemistry is mainly used to detect the expression of MMR-associated proteins in tumors, while PCR and secondary sequencing are used to detect abnormal expression of microsatellite markers in tumors. MMR or MSI testing is important for disease screening, diagnosis, therapeutic use and prognosis. For example, in colorectal cancer, pMMR/non-MSI-H type has a later age of onset, smaller tumor diameter, and more lymphatic metastases compared with dMMR/MSI-H type. Therefore MMR or MSI becomes a major indicator for clinical evaluation. If diagnosed with related diseases, testing and treatment should be performed as early as possible under the guidance of a physician.