An essential amino acid or indispensable amino acid is an amino acid that cannot be synthesized de novo(from scratch) by the organism being considered, and therefore must be supplied in its diet. The amino acids regarded as essential for humans are phenylalanine, valine, threonine, tryptophan, isoleucine, methionine, leucine, lysine, and histidine. Additionally, cysteine (or sulphur-containing amino acids), tyrosine (or aromatic amino acids), and arginine are required by infants and growing children. Essential amino acids are \\\\\\\"essential\\\\\\\" not because they are more important for protein synthesis or general homeostasis than the others, but because the body does not synthesize them. If they are not uptaken through diet, they will not be available for protein synthesis. In addition, the amino acids arginine, cysteine, glycine, glutamine, proline, serine and tyrosine are considered conditionally essential, meaning they are not normally required in the diet, but must be supplied exogenously to specific populations that do not synthesize them in adequate amounts. An example would be with the disease phenylketonuria (PKU). Individuals living with PKU must keep their intake of phenylalanine extremely low to prevent mental retardation and other metabolic complications. However, they cannot synthesize tyrosine from phenylalanine, so tyrosine becomes essential in the diet of PKU patients.