Below is an instruction from Human. Write a response.
    ### Instruction:
    How many people are affected by tetrahydrobiopterin deficiency ?
    ### Response:
    This condition is rare, affecting an estimated 1 in 500,000 to 1 in 1 million newborns. In most parts of the world, tetrahydrobiopterin deficiency accounts for 1 to 3 percent of all cases of elevated phenylalanine levels. The remaining cases are caused by a similar condition called phenylketonuria (PKU).
    