When there is a mutation in a gene that produces an enzyme, it can cause disease. Phenylketonuria or PKU is a rare inherited disorder that causes the build up of phenylalanine. This can lead to brain damage in individuals with this disorder if they don't eat a very protein-- restricted diet. The gene mutation doesn't allow the creation of the enzyme needed to break down Phenylalanine, an amino acid in proteins. In Tay Sachs disease, individuals with two copies of a recessive gene inherit a condition characterized by the progressive deterioration of nerve cells. Physical and mental problems ensue, along with seizures, paralysis and death by the age of four. When gangliosides accumulate in the brain cells of these individuals, the disease symptoms progress. This disease is due to insufficient activity of the enzyme hexosaminidase A which is a hydrolytic enzyme. Its job is to break down glycolipids. These lipids accumulate in the brain of affected individuals, in the absence of the enzyme. When a mutation causes an insufficient quantity of a needed enzyme, or the enzyme is not produced at all, a disease phenotype can be the result. Galactosemia is a disorder that affects the way the body processes the sugar galactose. In classic galactosemia or type I, infants must be treated with a low-galactose diet from birth or life threatening conditions can occur. These include lack of energy, feeding problems, failure to thrive, and delayed development. This condition is inherited when an individual inherits two recessive genes. Mutations in the GALT, GALK1, and GALE genes cause this condition because they provide the instructions to make enzymes for processing galactose in foods. Usually, this sugar is broken down to glucose which is used for energy or can be stored. In type I, the mutations almost completely stop the activity of the enzyme from the GALT gene. These are just three examples of how gene mutations affect the function of enzymes which can lead to disease.