Krug D, Baumann R, Budach W, et al. - Given a growing use of gene expression assays for decision-making about adjuvant chemotherapy in patients suffering from hormone receptor-positive, HER2-negative breast cancer and in view of the increasing availability of gene expression analyses, a question emerges that whether the results of gene expression assays can also be used to guide clinical decisions about adjuvant radiation. A correlation between results from gene expression assays and locoregional recurrence rates has been suggested in many studies. The incorporation of gene expression assays into the decision-making process for adjuvant radiation is not supported by the existing evidence on the possible predictive value of these assays for radiotherapy. The reasons for this include methodological weaknesses and limitations concerning patient selection, the nonrandomized design of all inquiries in terms of radiotherapy use, and limited availability of tissue from prospective trials. Therefore, extreme caution is advised when utilizing present knowledge for clinical indication of radiotherapy.