 This biomolecular "traffic jam" triggers the unfolded protein response, a hallmark of various neurodegenerative diseases.The effects of queuine deficiency have been studied in germ-free animals, which have no intestinal microbiota.After removal of all queuine from the diet, germ-free mice lose the ability to convert the dietary amino acid phenylalanine into tyrosine–a phenotype mimicking the human disease phenylketonuria.