Serine is a nonessential amino acid derived from glycine. Like all the amino acid building blocks of protein and peptides, serine can become essential under certain conditions, and is thus important in maintaining health and preventing disease. Low-average concentration of serine compared to other amino acids is found in muscle. Serine is highly concentrated in all cell membranes.  L-Serine may be derived from four possible sources: dietary intake; biosynthesis from the glycolytic intermediate 3-phosphoglycerate; from glycine ; and by protein and phospholipid degradation. Little data is available on the relative contributions of each of these four sources of l-serine to serine homoeostasis. It is very likely that the predominant source of l-serine will be very different in different tissues and during different stages of human development. In the biosynthetic pathway, the glycolytic intermediate 3-phosphoglycerate is converted into phosphohydroxypyruvate, in a reaction catalyzed by 3-phosphoglycerate dehydrogenase (3- PGDH; EC 1.1.1.95). Phosphohydroxypyruvate is metabolized to phosphoserine by phosphohydroxypyruvate aminotransferase (EC 2.6.1.52) and, finally, phosphoserine is converted into l-serine by phosphoserine phosphatase (PSP; EC 3.1.3.3). In liver tissue, the serine biosynthetic pathway is regulated in response to dietary and hormonal changes. Of the three synthetic enzymes, the properties of 3-PGDH and PSP are the best documented. Hormonal factors such as glucagon and corticosteroids also influence 3-PGDH and PSP activities in interactions dependent upon the diet. L-serine plays a central role in cellular proliferation. L-Serine is the predominant source of one-carbon groups for the de novo synthesis of purine nucleotides and deoxythymidine monophosphate. It has long been recognized that, in cell cultures, L-serine is a conditional essential amino acid, because it cannot be synthesized in sufficient quantities to meet the cellular demands for its utilization. In recent years, L-serine and the products of its metabolism have been recognized not only to be essential for cell proliferation, but also to be necessary for specific functions in the central nervous system. The findings of altered levels of serine and glycine in patients with psychiatric disorders and the severe neurological abnormalities in patients with defects of L-serine synthesis underscore the importance of L-serine in brain development and function.  (A3449).