Glycyl-tRNA synthetase, a class II aminoacyl-tRNA synthetase, catalyzes the synthesis of glycyl-tRNA, which is required to insert glycine into proteins. In a side reaction the enzyme also synthesizes dinuceloside polyphosphates, which probably participate in regulation of cell functions. Glycine is the smallest amino acid occurring in natural proteins, probably established as a protein component very early in evolution. Besides the amino and the carboxyl groups there is no functional group in the molecule. Alanine, the amino acid which is structurally most similar to glycine, possesses an additional methyl group as 'side chain'. Glycyl-tRNA synthetase is one of the few synthetases which exhibit different oligomeric structures in different organisms (alpha 2 beta 2 and alpha 2). The alpha 2 beta 2 enzymes exhibit similarities to PheRS (also an alpha 2 beta 2 enzyme). The alpha 2 forms belong to the subclass IIa enzymes with regard to sequence homologies. In eukaryotes the polypeptide is weakly associated with multienzyme complexes consisting of aminoacyl-tRNA synthetases. In the aminoacylation reaction a 'half-of-the-sites' mechanism as found for GlyRS from Bombyx mori is probably used by all glycyl-tRNA synthetases under in vivo conditions. Essentially, tRNAGly is recognized by GlyRS through standard identity elements in the anticodon region and in the acceptor stem. The last three facts may indicate that GlyRS is an enzyme which still possesses properties of a primordial aminoacyl-tRNA synthetase. Nine genes of glycyl-tRNA synthetases from six organisms have been sequenced. They encode synthetase subunits of chain lengths ranging from 300-700 amino acids. One crystal structure, that of the alpha 2 enzyme from Thermus thermophilus, has also been determined. The two subunits each possess three domains: the active site resembling that of aspartyl and seryl enzymes, a C-terminal anticodon recognition domain, and one domain which almost certainly interacts with the acceptor stem of tRNAGly. Antibodies against glycyl-RNA synthetase occur in the sera of patients suffering from polymyositis and interstitial lung disease.