The core2 beta(1,6)-N-acetylglucosaminyltransferase-I (C2GnT-I) is expressed by leukocytes and is involved in the synthesis of core2 O-glycans that carry sialyl-Lewis x (sLex) oligosaccharides. The core2-based sLex oligosaccharides (C2-O-sLex) have been demonstrated to be physiological selectin ligands that confer high affinity binding. The E-, P-, and L-selectins are adhesion proteins that direct leukocytes in the blood to lymphoid organs and sites of inflammation. They are also thought to be involved in the hematogenous dissemination of carcinoma cells expressing sialyl-Lewis glycans. Therefore, accumulation of data on structure-function relationships of this particular enzyme may represent an important part of investigations into pathologies involving selectins, such us inflammatory disorders and cancer progression. In this regard, studies of the intracellular distribution of C2GnT-I and its interaction with cognate substrates in vivo, as well as the knowledge of posttranslational modification (i.e., glycosylation, oligomerization, and proteolytic processing), may greatly aid in designing potential enzyme inhibitors. C2GnT-I fused to the green fluorescent protein is expressed to allow examination of the protein in living cells and to ease studies on structure-function relationships in vivo and in vitro.