About one in 10,000 children are born with phenylketoneuria (PKU). People with PKU are missing an enzyme that breaks down phenylalanine, one of the building blocks of the protein found in a normal diet. It can be serious if left untreated, causing mental impairment. However, PKU is treated successfully in by controlling levels of phenylalanine in the diet. All babies in the UK are now tested for PKU by a heel-prick test shortly after birth. If PKU is diagnosed babies are immediately given a special diet.