From OMIM
Mismatch repair cancer syndrome (MMRCS) is a rare autosomal recessive childhood cancer predisposition syndrome with 4 main tumor types: hematologic malignancies, brain/central nervous system tumors, colorectal tumors and multiple intestinal polyps, and other malignancies including embryonic tumors and rhabdomyosarcoma. Many patients show signs reminiscent of neurofibromatosis type I (NF1; 162200), particularly multiple cafe-au-lait macules (summary by Baas et al., 2013). Wimmer and Etzler (2008) provided a review of the mismatch cancer repair syndrome and suggested that the prevalence may be underestimated. Genetic Heterogeneity of Mismatch Repair Cancer Syndrome MMRCS2 (619096) is caused by mutation in the MSH2 gene (609309) on chromosome 2p21-p16. MMRCS3 (619097) is caused by mutation in the MSH6 gene (600678) on chromosome 2p16. MMRCS4 (619101) is caused by mutation in the PMS2 gene (600259) on chromosome 7p22. Patients with familial adenomatous polyposis (FAP; 175100), an autosomal dominant disorder that results from heterozygous mutations in the APC gene, may also develop brain tumors or extracolonic malignancies, resulting in a similar clinical phenotype. Heterozygous mutations in the MMR genes result in hereditary nonpolyposis colorectal cancer (see, e.g., HNPCC1, 120435).  http://www.omim.org/entry/276300

From MedlinePlus Genetics
Constitutional mismatch repair deficiency (CMMRD) syndrome is a rare disorder that greatly increases the risk of developing one or more types of cancer in children and young adults. The cancers that most commonly occur in CMMRD syndrome are cancers of the colon (large intestine) and rectum (collectively referred to as colorectal cancer), brain, and blood (leukemia or lymphoma).

Almost all people with CMMRD syndrome develop cancer before age 18, generally in late childhood. The age of diagnosis varies depending on the cancer type; brain cancers, leukemia, and lymphomas tend to occur at younger ages than colorectal cancer in people with CMMRD syndrome. It is estimated that 20 to 40 percent of people with CMMRD syndrome who develop cancer will develop another cancer later in life.

People with CMMRD syndrome may develop multiple noncancerous (benign) growths (adenomas) in the colon that are likely to become cancerous (malignant) over time. Brain cancers in CMMRD syndrome often involve certain cells called glial cells, causing gliomas or glioblastomas. The most common blood cancer in CMMRD syndrome is called non-Hodgkin lymphoma, which affects white blood cells. Other cancers that can occur in CMMRD syndrome include cancers of the small intestine, urinary tract, or uterine lining (endometrial cancer).

Many people with CMMRD syndrome develop features similar to those that occur in a condition called neurofibromatosis type 1. These features include changes in skin coloring (pigmentation), which are characterized by one or more flat patches on the skin that are darker than the surrounding area (café-au-lait spots). Some affected individuals have freckling or patches of skin that are unusually light in color (hypopigmented). Rarely, people with CMMRD syndrome will develop a feature of neurofibromatosis type 1 called Lisch nodules, which are benign growths that often appear in the colored part of the eye (the iris). Lisch nodules do not interfere with vision. Some people with CMMRD syndrome are initially misdiagnosed with neurofibromatosis type 1.  https://medlineplus.gov/genetics/condition/constitutional-mismatch-repair-deficiency-syndrome