The metabolic adaptations that maintain plasma amino acid levels in mice with genetic loss of Slc6a19 despite broad aminoaciduria do not extend to the toxic levels of Phe in the Pahenu2 mouse model of PKU; there, genetic loss of Slc6a19 selectively lowered the highly elevated plasma Phe levels while other plasma amino acids were largely unaffected (14). Here, SLC6A19 is linked to phenylketonuria.