of a tumor can thus be prevented by the timely surgical removal of the ki ras mutated polyps up till now there have not been any such diagnoses because of the absence of a system of detection which can be used routinely pursuant to the invention at least six different oligonucleotides which have a defined complementarity to known mutatively changed sequence sections of the ki ras gene are used for investigating the ki ras target sequence with that the possibility exists of detecting unambiguously more than 80 of all ki ras mutations associated with a malignant phenotype finally the invention will be described in greater detail by means of an example transfer approximately 300 to 500 mg of a stool sample to a 2 0 ml eppendorf reaction vessel add 350 u03bcl of a washing solution nacl tris edta and swirl centrifuge for 2 minutes at 14 000 rpm and transfer the supernatant into a new 2 0 ml eppendorf reaction vessel add 250 u03bcl of a solution of activated charcoal shake sample for 10 minutes centrifuge for 2 minutes at 14 000 rpm and transfer the supernatant into a new 1 5 ml eppendorf reaction vessel centrifuige for 1 minute at 14 000 rpm and transfer the supernatant into a new 2 0 ml eppendorf reaction vessel add 1 ml of buffer guanidine isothiocyanate dtt sodium citrate to lyse cells and incubate for 30 minutes at room temperature add.