The aspartame metabolite phenylalanine is an essential amino acid which occurs naturally in the breast milk of mammals; however high levels of phenylalanine are a health hazard to those born with phenylketonuria (PKU), a metabolic disorder caused by an inherited mutation in the phenylalanine hydroxylase (PAH) gene which prevents phenylalanine from being metabolized correctly. The gene discussed is PAH; the disease is metabolic disease.