 Additionally, the aspartate kinase-homoserine dehydrogenase gene is primarily expressed in actively growing, young plant tissues, particularly in the apical and lateral meristems.Mammals lack the enzymes involved in the aspartate metabolic pathway, including homoserine dehydrogenase.As lysine, threonine, methionine, and isoleucine are made in this pathway, they are considered essential amino acids for mammals.