Phosphomannomutase 2, also known as PMM2 and CDG1, belongs to theeukaryotic PMM family. Phosphomannomutase 2 catalyzes the isomerization of mannose 6-phosphate to mannose 1-phosphate. Mannose 1-phosphate is a precursor to GDP-mannose necessary for the synthesis of dolichol-P-oligosaccharides. GDP-mannose can transfer its small sugar molecule called mannose to the growing oligosaccharide chain. Once the correct number of small sugar molecules are linked together to form the oligosaccharide, it can be attached to a protein. Phosphomannomutase 2 is also required for a number of critical mannosyl transfer reactions. Mutations in PMM2 gene have been shown to cause defects in the protein glycosylation pathway manifest as carbohydrate-deficient glycoprotein syndrome type I.