Dr. Asbjørn Følling was a Norwegian medical scientist whose work paved the way for early diagnosis and treatment of phenylketonuria (PKU also known as Følling's disease). Tipped by the mother of two PKU children about the unusual smell of their urine, he established that the substance causing the green and odorous urine was phenylpyruvic acid. Følling eventually discovered that the body of these people lack the ability to break down the amino acid fenylananin in the blood. Phenylalanine is one of the 20 amino acid building blocks used by the body to construct proteins, including the proteins that act as catalysts – the enzymes. Such an association between a defect in the metabolic process of the body and the developmental retardation was previously unknown. Correctly treated with a strict modern diet, children born with PKU develop normally. It is a very rare disease, with a frequency of about one in 12.000 newborns. A blood test done on the newborn child now safely demonstrates the presence of the disease. In many countries it is mandatory to make this test, which has saved thousands of children from becoming mentally retarded. Følling died in 1973. He is considered by many the most important medical scientist not to receive the Nobel Prize for Physiology or Medicine.