Lec3 Chinese hamster ovary (CHO) cell glycosylation mutants have a defect in sialic acid biosynthesis that is shown here to be reflected most sensitively in reduced polysialic acid (PSA) on neural cell adhesion molecules. To identify the genetic origin of the phenotype, genes encoding different factors required for sialic acid biosynthesis were transfected into Lec3 cells. Only a Gne cDNA encoding UDP-GlcNAc 2-epimerase:ManNAc kinase rescued PSA synthesis. In an in vitro UDP-GlcNAc 2-epimerase assay, Lec3 cells had no detectable UDP-GlcNAc 2-epimerase activity, and Lec3 cells grown in serum-free medium were essentially devoid of sialic acid on glycoproteins. The Lec3 phenotype was rescued by exogenously added N-acetylmannosamine or mannosamine but not by the same concentrations of N-acetylglucosamine, glucosamine, glucose, or mannose. Sequencing of CHO Gne cDNAs identified a nonsense (E35stop) and a missense (G135E) mutation, respectively, in two independent Lec3 mutants. The G135E Lec3 mutant transfected with a rat Gne cDNA had restored in vitro UDP-GlcNAc 2-epimerase activity and cell surface PSA expression. Both Lec3 mutants were similarly rescued with a CHO Gne cDNA and with CHO Gne encoding the known kinase-deficient D413K mutation. However, cDNAs encoding the known epimerase-deficient mutation H132A or the new Lec3 G135E Gne mutation did not rescue the Lec3 phenotype. The G135E Gne missense mutation is a novel mechanism for inactivating UDP-GlcNAc 2-epimerase activity. Lec3 mutants with no UDP-GlcNAc 2-epimerase activity represent sensitive hosts for characterizing disease-causing mutations in the human GNE gene that give rise to sialuria, hereditary inclusion body myopathy, and Nonaka myopathy.