The PKU-associated enzyme deficiency was determined biochemically in the 1950s—long before the PAH-encoding gene was mapped to human chromosome 12 and cloned in 1983. Specifically, Dr. Willard ...

A rare disease in the US is one in 1,500, or fewer than 200,000 people, and an ultra-rare disease is one in 50,000 or fewer than than 6,000 people.