amino acids the substances of which protein is made. Twenty-four of them are known to be involved in the synthesis of protein, of which 20 are important. These are glycine, alanine, proline, valine, leucine, isoleucine, methionine, phenylalanine, tryptophan, aspartic acid, glutamic acid, tyrosine, cysteine, serine, threonine, asparagine, glutamine, histidine, lysine, and arginine. The names in italics are those of the 10 ‘essential’ amino acids for humans. These cannot be made in the body, and have to form part of the diet. The others can be made by altering other amino acids. Other animals have different sets of essential acids.