HNPCC is a hereditary cancer syndrome of several organs but more particularly the colon and the uterus. It is characterized by microsatellite instability (MSI) related to mutations of DNA mismatch repair (MMR) genes. The role of the pathologist in the detection of colorectal cancer with MSI phenotype is important. Among the available tests to detect MSI, immunohistochemistry appears being a sensitive, specific and inexpensive test. The detection of HNPCC syndrome by the genetic test among the MSI cancers implies a coordination of the structures of health and requires a follow-up of the patients, during and after the genetic test. Ethical dimension related to these hereditary cancers must be considered without constituting a brake for their detection