Gene expression arrays are currently used to guide therapy decisions in breast cancer. The indications for gene expression tests are especially important in the group of hormone receptor negative, HER2 positive tumors to decide whether endocrine therapy alone is sufficient or additional chemotherapy is necessary. In this group of luminal tumors conventional clinicopathological parameters are often not suitable to select patients who would benefit from an endocrine therapy alone. Gene expression tests can provide additional information and, therefore, support decision-making and avoid unnecessary chemotherapy. There are a variety of test systems available which poses the questions of which tests should be selected for which patients and how the test results should be evaluated in a direct comparison. This report provides information about three currently available gene expression tests (i.e. OncotypeDx®, Endopredict® and PAM50/Prosigna®), comments on similarities and differences and discusses the impact on therapy decisions. The focus of this article is on a discussion of clinical studies that have compared the different molecular tests in the same clinical study cohort. These investigations allow a first comparative evaluation of the various assays for breast cancer.