Hereditary nonpolyposis colorectal cancer is an autosomal dominant condition caused by highly penetrant gene mutations. It is characterized by increased susceptibility for a specific group of cancer, mainly colorectal cancer. The syndrome originates from the inheritance of mutations in DNA mismatch repair genes. The most commonly affected genes in hereditary nonpolyposis colorectal cancer are hMLH1 and hMSH2. Their deficient expression renders the cell susceptible to the accumulation of many molecular defects, a condition which can be evaluated by the instability in sections of base repeats in the genoma known as microsatellite instability. The molecular detection of hereditary nonpolyposis colorectal cancer is possible in most of the highly suspicious cases. Genetic tests for hereditary nonpolyposis colorectal cancer also allow characterization of the individual that bears the mutation within a family. The high cost and restricted availability of these tests hamper their use for every person presenting colorectal cancer. Due to this fact, some clinical criteria have been developed by a hereditary nonpolyposis colorectal cancer international organization to select families with a high probability of carrying the mutation. Once families at risk are identified, they are encouraged to join a screening program that aims at early detection of hereditary nonpolyposis colorectal cancer-related cancers, increasing the possibility of its prevention and early detection.