Dwarfism is often caused by the person inheriting an allele (a mutant gene), which produces a defect in development. Defects caused by genetics are the main kind of congenital defects, and usually cannot be cured. The most common cause of dwarfism is a defect in bone development called "achondroplasia", in which the limbs are short in proportion to the body. This accounts for 70% of cases. Extreme shortness, but with body parts of normal proportions, may be caused by growth hormone deficiency, once known as "pituitary dwarfism". There are more than 200 other conditions which may cause dwarfism.