Below is an instruction from Human. Write a response.
    ### Instruction:
    How many people are affected by phenylketonuria ?
    ### Response:
    The occurrence of PKU varies among ethnic groups and geographic regions worldwide. In the United States, PKU occurs in 1 in 10,000 to 15,000 newborns. Most cases of PKU are detected shortly after birth by newborn screening, and treatment is started promptly.
    