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Computer graphic representation of the haemoglobin molecule from a human red blood cell, or erythro- cyte. The cylindrical shapes are the polypeptide chains (protein), of which there are four in two identical pairs. The protein combines with four bead-like discs which contain the iron pigment, haem. Haemoglobin is the medium by which oxygen is transported within the body. (KEYSTONE/SCIENCE PHOTO LIBRARY/LABORATORY OF MOLECULAR BIOLOGY, MRC)