Other enzymes that have been described for use in enzyme replacement therapy are phenylalanine hydroxylase (for phenylketonuria [47,48,49,50]), adenosine deaminase (for severe combined immunodeficiency with impaired humoral and cellular immune response [27,51,52,53,54,55]) and thymidine phosphorylase (for the treatment of mitochondrial neurogastrointestinal encephalomyopathy (MNGIE) [56,57]). Here, PAH is linked to mitochondrial neurogastrointestinal encephalomyopathy.