In this research, we examined which genes are involved in N-linked sialylation in Chinese Hamster Ovary (CHO) cells using siRNA knockdown approaches. Three genes from the sialyltransferase family (ST3GAL3, ST3GAL4 and ST3GAL6) were chosen as knockdown targets with siRNA applied to reduce their expression. Single, double and triple gene knockdowns were investigated, and the reduction levels of sialylation on the total cell lysate were monitored by enzyme-linked lectin absorption assays (ELLA) and sialic acid quantification with high performance liquid chromatography (HPLC). All transfection groups showed effective reduction in 2,3-linked sialylation whereas the trend of reduction levels of triple siRNA transfection outweighed both the dual siRNA groups and single siRNA transfection groups. Next, this transfection approach was applied to CHO cells producing erythropoietin (EPO). Quantification of EPO sialylation showed similar result to total cell lysate except that the ST3GAL4 siRNA transfection exhibited the largest reduction according to the HPLC analysis as compared with other single siRNA transfections. Finally, the N-glycan released from the EPO transfected with ST3GAL4 siRNA showed a prominent reduction in sialyation level among the single siRNA transfections. From these experiments, we concluded that each of these three genes were involved in N-linked sialylation and ST3GAL4 may play the critical role in glycoprotein sialylation of recombinant proteins such as EPO. (C) 2015 Elsevier Inc. All rights reserved.