Phenyl Ketone Urea...... inability to process phenylalanine properly, body ends up dumping it in the brain as a toxic residue, bloody horrible, they test all babies for it (part of the reason they take blood from newborns) and if you've got it you have to avoid anything with "contains a souce of phenylalanine" on the label. It's just another amino acid to the rest of us, that's why they don't have to label all of them (all proteins are made from them).