cytomorphologic methods e g 1 to 5 at this time up to 10 10 malignant cells can still remain in the patient they represent the minimal residual disease mrd detection of low frequencies of residual malignant cells allows a longer follow up of the tumor burden during chemotherapy and thus permits to better appreciate the sensitivity of leukemia cells to treatment it is now established that the level of mrd represents a powerful prognostic factor for final outcome besides the detection of an increase of the mrd level enables to anticipate impending relapse the method provided in the present invention allows to discriminate between the presence of normal proteins and an aberrant fusion protein at the single cell level as an example of this method we will describe the preparation of a probe set for the detection of the tel aml1 fusion protein also described is a method using this probe set to detect the presence of tel aml1 fusion protein in all cells at the time of diagnosis and during follow up to detect the level of mrd preferably a probe set according to the invention comprises a set of two fluorochrome conjugated antibodies each antibody additionally provided with a reactive group methods of producing an antibody are known to those skilled in the art for example to obtain a polyclonal antibody a laboratory animal is immunized with an immunogen such as a recombinant protein or.