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A view of the transmembrane domain of the nicotinic acetylcholine receptor from Torpedo marmorata, with proposed coordinates for embedded cholesterol molecules. Each subunit contains four membrane-spanning helices (M1 purple, M2 green, M3 blue, M4 cyan) and three bound cholesterol molecules (yellow, orange and red). The image is based on the crystal structure PDB ID: 2BG9 from Ref. (Unwin, 2005).
