When a woman is diagnosed with breast cancer, the difficult question arises as to which type of treatment is the right one. Gene expression testing is one of the methods used by doctors to help make a prognosis about the course of the disease and, based on this, to select a suitable therapy. However, the reliability of these tests has not been fully established. Scientists from Leipzig University and the Pathologie Hamburg-West institute have now used machine learning to analyse large amounts of data on this question and found that gene expression signatures offer a high degree of certainty in prognosis, but not complete certainty.