PMM1 (phosphomannomutase 1) is a cytosolic enzyme that catalyzes the reversible conversion of mannose 6-phosphate to mannose 1-phosphate 1, playing a critical role in GDP-mannose and dolichol-phosphate-mannose synthesis required for protein N-linked glycosylation 2. The enzyme is a 262-amino acid protein with ~29 kDa molecular mass 2 and forms a phosphoaspartyl intermediate during catalysis 3. PMM1 is distinct from the related isozyme PMM2, which is encoded on chromosome 22; PMM1 itself maps to chromosome 22 4. Tissue expression analysis reveals particularly high expression in brain tissue compared to PMM2 1, though both isoforms show overlapping brain distribution 5. PMM1 also possesses a secondary degradative function for glucose-1,6-bisphosphate in the brain under IMP influence 6. While mutations in PMM2 cause congenital disorders of glycosylation type Ia (CDG-Ia), no disease has been definitively associated with PMM1 deficiency 4. Recent evidence suggests PMM1 may have additional substrate specificity or functions beyond phosphomannomutase activity in vivo 5. PMM1 serves as a reliable housekeeping gene reference for gene expression studies in hepatic tissue 7.