(D) Fragile X # HNPCC is caused by mutations in one of several different mismatch repair (MMR) genes including MutS homologue 2 (MSH2) and MutL homologue 1 (MLH1). These enzymes are involved in the detection of nucleotide mismatches and in the recognition of slipped-strand trinucleotide repeats. Germline mutations in these genes lead to microsatellite instability and a high mutation rate in colon cancer. This syndrome is characterized by autosomal dominant transmission of colon cancer, young age (50 years) of presentation, predisposition to lesions in the proximal large bowel, and associated malignancies such as uterine cancer and ovarian cancer. Fanconi's anemia is also associated with an increased risk of multiple acquired genetic abnormalities. It is characterized by diverse congenital anomalies and a strong predisposition to develop aplastic anemia and acute myelogenous leukemia. Cells from these patients are susceptible to chromosomal breaks caused by a defect in genetic recombination. A least eight different complementation groups have been identified, and several loci and genes associated with Fanconi's anemia have been mapped or cloned. Ataxia telangiectasia causes large telangiectatic lesions of the face, cerebellar ataxia, immunologic defects, and hypersensitivity to ionizing radiation. The discovery of the ataxia telangiectasia mutated (ATM) gene reveals tha: it is homologous to genes involved in DNA repair and control of cell cycle checkpoints. Mutations in the ATM gene give rise to defects in meiosis as well as increasing susceptibility to damage from ionizing radiation. Trinucleotide expansion was first recognized as a cause of the fragile X syndrome, one of the most common causes of mental retardation.