Finally, the use of a genetically modified lactic acid bacteria, as an in situ (i.e. gut) expression system to produce a recombinant phenylalanine hydroxylase (PAH) for the treatment of PKU, showed promising results in the evaluation of a new strategy to facilitate the oral administration of recombinant enzymes for the treatment of IEM [75]. The gene discussed is PAH; the disease is phenylketonuria.