Many genetic disorders are caused by mutations (or changes) in gene alleles. Mutations can cause development to go wrong, or a simple biochemical deficiency (the body does not have enough of an important chemical). In the case of a chemical deficiency, the problem may be curable. For example, phenylketonuria was a cause of mental retardation. Doctors came up with a simple test to find which babies have this condition. They found that if these children got treated and followed a strict diet, they could get enough of the chemical they needed. Phenylketonuria now causes few cases of mental retardation.