anomaly in the amount of serine, 2-amino-3-hydroxypropanoic acid, a non-essential alpha-amino acid that is alanine substituted at position 3 by a hydroxy group; serine participates in the biosynthesis of purines and pyrimidines, is the precursor to glycine and cysteine and to numerous metabolites including sphingolipids and folate, and is often a hydrogen donor in enzymes, such as trypsin and chymotrypsin, playing a key role in their function as catalysts