ST8SIA4 encodes an alpha-2,8-sialyltransferase that catalyzes the transfer of sialic acid onto terminal sialic acids of N-linked glycans, particularly on neural cell adhesion molecule (NCAM1), fetuin-B (FETUB), and alpha-2-HS-glycoprotein (AHSG), generating polysialic acid chains. Alpha-2,3-linked sialic acids serve as preferred acceptor substrates compared to alpha-2,6-linked variants. The enzyme localizes to the Golgi apparatus and participates in ganglioside and glycoprotein biosynthesis with roles in nervous system development. In disease contexts, ST8SIA4 exhibits opposing functions depending on tissue type. In breast cancer, elevated ST8SIA4 drives polysialic acid accumulation on epithelial membranes, promoting an acidic, immunosuppressive tumor microenvironment that facilitates metastasis and resistance to checkpoint inhibitors such as anti-PD1 therapy 1. Conversely, in follicular thyroid carcinoma, ST8SIA4 suppression enhances cancer progression through miR-146a/b-mediated upregulation of PI3K-AKT-mTOR signaling 2. In chr5 myeloid leukemia, ST8SIA4 overexpression correlates with multidrug resistance; downregulation of ST8SIA4 via miR-181c restores chemosensitivity to adriamycin 3. In remyelination after demyelinating injury, ST8SIA4 delays oligodendrocyte differentiation and impairs myelin repair, whereas ST8SIA2-mediated polysialylation promotes recovery 4. A sialyltransferase inhibitor, 3Fax-Peracetyl Neu5Ac, neutralizes the immunosuppressive microenvironment in BRCA1-deficient breast cancers and sensitizes tumors to immune checkpoint blockade 1.