MPI (mannose phosphate isomerase) is a cytosolic enzyme that catalyzes the interconversion of fructose-6-phosphate and mannose-6-phosphate, playing a critical role in GDP-mannose biosynthesis 1. This metabolic function is essential for supplying D-mannose derivatives required for eukaryotic protein glycosylation reactions. Mutations in the MPI gene cause congenital disorder of glycosylation type 1B (CDG-1B), a potentially treatable condition characterized by hypoglycosylation of serum and tissue glycoproteins 1. CDG-1B presents as the gastrointestinal form of congenital disorders of glycosylation, manifesting with congenital hepatic fibrosis and protein-losing enteropathy due to impaired glycoprotein synthesis 1. The MPI gene spans approximately 5 kilobases across 8 exons; disease-causing mutations include missense mutations, splice variants, and insertions that can destabilize transcripts 1. Clinical significance stems from CDG-1B being one of the few treatable glycosylation disorders, potentially responsive to mannose supplementation therapy. Understanding MPI function and mutations is important for genetic diagnosis and therapeutic intervention in this rare inherited metabolic disorder.