Haemoglobin S. Molecular model of the mutant form of haemoglobin (haemoglobin S) that causes sickle cell anaemia. This is deoxyhaemoglobin S, the molecule in its non-oxygen bound state. Haemoglobin transports oxygen around the body in red blood cells. It consists of four globin proteins (amino acid chains), each wrapped around a haem group. Sickle cell anaemia is an inherited blood disease where the mutant form of haemoglobin causes the blood cells to become sickle-shaped, rather than round. Sickle cells cannot move through small blood vessels as easily as normal cells and so can cause blockages. This prevents oxygen from reaching the tissues, causes severe pain and organ damage.