Final answer:
Hereditary nonpolyposis colon cancer is caused by a defect in genes that encode for DNA mismatch repair proteins. A key finding in these patients is the presence of microsatellite instability (MSI).
Step-by-step explanation:
Hereditary nonpolyposis colon cancer, also known as Lynch syndrome, is caused by a defect in genes that encode for DNA mismatch repair proteins such as MLH1, MSH2, MSH6, and PMS2. These proteins are responsible for repairing errors that occur during DNA replication. When these genes are defective, DNA errors can accumulate, leading to the development of colon cancer.
A key finding in patients with hereditary nonpolyposis colon cancer is the presence of microsatellite instability (MSI). MSI is a result of the defective DNA mismatch repair system and refers to small changes in the length of repetitive sequences in the DNA. It is used as a diagnostic marker for Lynch syndrome.