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Sialyl Tn antigen (Siaα2-6GalNAca1-O-Ser/Thr) is such a kind of important sialylated structure,which has been reported to correlate with the invasive and metastatic growth of carcinoma cells and considered as a specific tumor-associated antigen for vaccination(1,2).We have previous reported the fusion sialyltransferase Psp26ST(15–501)-His6 which can be used in the chemoenzamytic syntheis of STn antigen(3).Here,mutagenesis studies around its binding pocket and catalytic center have been carried out in order to improve its sialyltransferase efficiency toward α-GalNAc containing acceptors for the STn synthesis.Ten mutants have been expressed,purifed compared with the wild type.