MPI is a member of the mannose-6-phosphate isomerase type 1 family. Although MPI is expressed in all tissues, it can be found more abundantly in heart, brain and skeletal muscle. Localized to the cytoplasm, MPI exploits zinc as a cofactor and catalyzes the interconversion of fructose-6-phosphate and mannose-6-phosphate. Mutations in the MPI gene are the cause of carbohydrate-deficient glycoprotein syndrome, type Ib.