B3GALT5 (β-1,3-galactosyltransferase 5) is a key glycosyltransferase enzyme. It is responsible for catalyzing the synthesis of the terminal glycan structure, the sialyl Lewis X antigen. This antigen serves as an important ligand for selectin family proteins, mediating cell adhesion in both physiological and pathological processes. Its core function lies in regulating cell migration. In immunology, it influences lymphocyte homing; in oncology, its aberrant upregulation promotes the synthesis of sLe^x, leading to the adhesion of cancer cells to vascular endothelium via the "selectin-ligand" recognition system. This represents a critical initial step in tumor hematogenous metastasis, such as to the liver or lungs.