Another important globular protein that has received significant experimental and theoretical investigation is sickle hemoglobin. The reason it has received so much attention is that it is related to sickle cell anemia. This disease results from the polymerization of sickle hemoglobin molecules via a complex two-step homogeneous/heterogeneous nucleation process to form fibers in solution. Although this non-equilibrium fiber state eventually will form a crystalline state, for all practical purposes it is a long-lived pseudo-equilibrium state. This polymerization of sickle hemoglobin molecules does not occur in globular proteins such as lysozyme or the γ-crystallins. Thus its crystal nucleation process differs from most known globular proteins.

Sickle cell anemia is a genetic disorder that affects red blood cells, which become hard and pointed instead of soft and round. More than 70 000 residents of the USA have sickle cell anemia; about 250 000 babies are born with this disease each year worldwide. The genetic nature of the disease is that two genes for the sickle hemoglobin must be inherited in order to have the disease. If only one mutated gene is inherited and another gene is normal, the person has a so-called “sickle cell trait.” People who have this sickle cell trait will not develop the disease, but they can pass the sickle cell gene to their children.