Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Hartnup disease ?
    ### Response:
    Hartnup disease is a metabolic disorder characterized by abnormal transport of certain amino acids in the kidney and gastrointestinal system. It is a type of aminoaciduria. The condition may be diagnosed based on the results of newborn screening tests. Most people with the condition have no symptoms (asymptomatic).
    