Relying solely on DNA sequence for PKU diagnosis is insufficient because it does not perfectly correlate with disease phenotype.

“reading out the DNA sequence we're good at we're not perfect at and so being able to decide based on your DNA sequence alone for phenyalanine hydroxy the relevant Gene we're not able to perfectly make that onetoone correlation about whether or not you'll be Beyond a threshold of disease in terms of phenyalanine levels so we do need in this case I'll call the phenotype the level of phenyalanine levels in the blood we do need to have that phenotype before we get to the phenotype of intellectual disabilities which is what we're trying to prevent”