In the 9 October 2009 issue, Science published the Research Article “Reactome array: Forging a link between metabolome and genome” by A. Beloqui et al. (1). Science is publishing this Editorial Expression of Concern to alert our readers to the fact that serious questions have been raised about the methods and data presented in this article. The questions focus in particular on the synthesis of the dye-labeled metabolites that are central to the microarray technique. In addition, the spectroscopic data the authors cite in support of their claim were not posted to the Bangor University School of Biological Sciences Web site at the time of publication, despite the authors' indication in the Supporting Online Material that the data would be so posted. In response to inquiries from Science, the authors have provided new descriptions of the synthetic methods that differ substantially from those in their published article. Based on our original concerns and the authors' response, Science has requested evaluation of the original data and records by officials at the authors' institutions: These officials have agreed to undertake this task.