Predictive medicine: incipient reality or fata morgana?

The success of targeted therapy depends on reliable biomarkers that indicate the presence and accessibility of the target molecules with which the therapeutic drug will interact. In principle, this is independent of whether the drug is a small molecule, such as kinase inhibitors, or a therapeutic antibody, such as trastuzumab or cetuximab. The functional system of target-drug interaction is described as 'predictive medicine', one of the great hopes in the treatment of cancer. The major challenges are the precise prediction of the patient's individual prognosis, of the probability of metastases, and of individual therapeutic response. The questions, challenges, and possible answers related to this complex issue as well as the prerequisites for the development of more efficient and less toxic cancer therapy are discussed.