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We have engineered two Chinese hamster ovary cell lines secreting different recombinant glycoproteins to express high levels of human β1, 4-galactosyltransferase (GT, EC 2.4. 1.38) and/or α2, 3-sialyltransferase (ST, EC 2.4. 99.6). N-linked oligosaccharide structures synthesized by cells overexpressing the glycosyltransferases showed greater homogeneity compared with control cell lines. When GT was overexpressed, oligosaccharides terminating with GlcNAc were significantly reduced compared with controls, whereas overexpression of ST resulted in sialylation of≥ 90% of available branches. As expected, GT overexpression resulted in reduction of oligosaccharides terminating with GlcNAc, whereas overexpression of ST resulted in sialylation of≥ 90% of available branches. The more highly sialylated glycoproteins had a significantly longer mean residence time in a rabbit model of pharmacokinetics. These …