ST8SIA4 and cancer: It has been known that NCAM-polySia expression on cancer cells is catalyzed by two polysialyltransferases (polySTs), ST8SiaIV and ST8SiaII; specifically, two polybasic motifs, Polybasic Region (PBR) and Polysialyltransferase Domain (PSTD) within each polyST, have been found to be critically important for polyST activity based on recent mutation and molecular modeling analyses [16,23].