Phosphomannomutase 2 (PMM2) is an enzyme that is a member of the highly variable methyltransferase superfamily. PMM2 is a cytoplasmic protein and catalyzes the isomerization of mannose 6-phosphate to mannose 1-phosphate.In addition, PMM2 involved in the synthesis of the GDP-mannose and dolichol-phosphate-mannose that required for a number of critical mannosyl transfer reactions. Defects in PMM2 can results in congenital disorder of glycosylation type 1A (CDG1A). Congenital disorders of glycosylation are metabolic deficiencies in glycoprotein biosynthesis that usually cause severe mental and psychomotor retardation.