Describes cells that have mutations (changes) in certain genes that are involved in correcting mistakes made when DNA is copied in a cell. Mismatch repair (MMR) deficient cells usually have many DNA mutations, which may lead to cancer. MMR deficiency is most common in colorectal cancer, other types of gastrointestinal cancer, and endometrial cancer, but it may also be found in cancers of the breast, prostate, bladder, and thyroid. MMR deficiency may also be found in an inherited disorder called Lynch syndrome. Knowing if a tumor is MMR deficient may help plan treatment or predict how well the tumor will respond to treatment.