Below is an instruction from Human. Write a response.
    ### Instruction:
    How many people are affected by hypermethioninemia ?
    ### Response:
    Primary hypermethioninemia that is not caused by other disorders or excess methionine intake appears to be rare; only a small number of cases have been reported. The actual incidence is difficult to determine, however, since many individuals with hypermethioninemia have no symptoms.
    