Midwife performing a Guthrie test on a newborn baby. A blood sample, taken from a heel stab, is tested for phenylketoneuria, a rare, hereditary defect of protein metabolism. The defect causes an excess of the amino acid phenylalanine in the blood, which, if untreated, damages the nervous system & leads to severe mental retardation. The screening of newborn infants enables the condition to be detected early enough for dietary treatment to prevent any brain damage. The baby is given a diet consisting of proteins from which phenyl- alanine has been removed. The gene responsible for phenylketoneuria is recessive and the condition affects around one in 20,000 babies.