Each year 3500 new cases of colorectal cancer (13% of total cancer cases) are registered in Switzerland. A yet unknown proportion of these cancers is associated with recently discovered gene defects in one of at Peast 4 genes participating together in an essential process. The function of these genes aims at the correction of certain erroneous hereditary informations that may occur when bases are not or mis-aligned. Mutations leading to anomalies in the expression of one of these genes favour strongly the development of certain early carcinomas, because they lead to an accelerated accumulation of further mutations expected to trigger carcinogenesis. It is estimated that about 2-3/1000 of the population carries a gene error typically manifested as the so called Lynch Syndrome, that concerns not only the colon but also the uterus, ovaries, the urogenital tract and diverse parts of the gastro-intestinal tract. The first observation which led the identification of these genes was a genetic instability within tumor cells showing a distinctly increased mutation rate in many different locations of the genome. Molecular identification of a factor predisposing to malignancy in one of these repair genes permits on one hand to abolish unnecessary investigations in members of families at risk that could be identified as non-carriers of this incriminating mutation and on the other hand to concentrate medical attention on carriers.