[When and how to investigate a mutation predisposing to the Lynch syndrome].

Lynch Syndrom (or Hereditary Non-polyposic Colorectal Cancer (HNPCC)) can be described as the presence of an autosomic dominant mutation predisposing to early colorectal cancer. Suggestive familial history and young patient with colorectal cancer should undergo thorough investigation. In Switzerland, investigation will first attempt to show microsatellites instability in fumoral cells, then mutation in blood. If positive, direct progeny and family should be tested. Those who are mutation free may be followed as normal population. The others should have more frequent colorectal and gynecologic follow-up. Surgical treatment will attempt to prevent synchrone or metachrone colorectal cancer by total colectomy. Preventive colectomy is attractive but controversial.