‘Haemoglobin’ explains the key role played by this protein: binding oxygen in the lungs and carrying it around the blood circulation, where it is offloaded to every cell in the body. Haemoglobin has also played a key role in the history of our understanding of all proteins and the science of biochemistry. The complex, three-dimensional quaternary structure of haemoglobin was identified in 1960 by Max Perutz. A haemoglobin tetramer contains four haem groups and can bind four molecules of oxygen. Chemical interaction of oxygen with haem iron is explained along with diseases—such as sickle cell disease, thalassaemias, and haemoglobinopathies—caused by changes in the globin part of the protein.