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COVER Structure of the human β2-adrenergic receptor (red) embedded in a lipid membrane and bound to a diffusible ligand (green), with cholesterol (yellow) between the two receptor molecules. A cartoon of the lipidic cubic phase used for crystallization of the receptor is shown in the background. See pages 1258 and 1266. Image: Yekaterina Kadyshevskaya and the Stevens Laboratory, Scripps Research Institute