GMPPB is a 360 amino acid protein that belongs to the transferase hexapeptide repeat family and is involved in protein modification pathways. Functioning as a GDP-mannose pyrophosphorylase, GMPPB enzymatically catalyzes the conversion of mannose-1-phosphate and GTP to GDP-mannose and a free phosphate, a reaction that is involved in the production of N-linked oligosaccharides. Defects in the gene encoding GMPPB that cause errors in the glycosylation pathway may lead to congenital disorders of glycosylation (CDG). CDGs are multisystemic diseases that often involve both the central and peripheral nervous systems and are often characterized by endocrine and coagulation disorders. GMPPB is expressed as two isoforms due to alternative splicing events.