Computer graphics of the haemoglobin molecule in the diseased state of sickle cell anaemia. Sickle cell is associated with a single mutation in the haemoglobin; the large protein molecule in red blood cells that binds and transports oxygen from the lungs to the tissues. The mutation is a change at residue 6 of the beta chain from glutamic acid to valine. Haemoglobin has two beta chains both with the valine mutation, but only one can be seen here as indicated. As a result of this mutation the deoxygenated haemoglobin forms aggregates in the red blood cells, preventing them from passing through the capillaries.