Spherocytosis is an auto-hemolytic anemia (a isease of the blood) characterized by the production of red blood cells (RBCs), or erythrocytes, that are sphere-shaped, rather than donut-shaped or, more specifically, bi-concave disk shaped. It is caused by a molecular defect in one or more of the proteins of the red blood cell cytoskeleton, including, spectrin, ankyrin, Band 3, or Protein 4.2. Because the cell skeleton has a defect, the blood cell contracts to its most surface-tension efficient and least flexible configuration, a sphere, rather than the more flexible donut-shape. The sphere-shaped red blood cells are known as spherocytes.