Below is an instruction from Human. Write a response.
    ### Instruction:
    How many people are affected by hereditary xanthinuria ?
    ### Response:
    The combined incidence of hereditary xanthinuria types I and II is estimated to be about 1 in 69,000 people worldwide. However, researchers suspect that the true incidence may be higher because some affected individuals have no symptoms and are never diagnosed with the condition.
    