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A two-color microarray experiment in which one nucleic acid sample is labeled with one color (e.g., green), a second sample is labeled with another color (e.g., red), the two samples competitively hybridize to the microarray, and the color intensity (e.g., red/green ratio) of each spot is measured. In this issue, Oliver Rando presents methods to label RNA samples for microarrays. The illustration is from an introduction to microarrays (doi: 10.1101/pdb.top096388).