This enzyme catalyzes O-glycan branch synthesis of the core 2 and core 4 type in mucins and controls expression of core 2 branched oligosaccharides and I antigens on the cell surface.This enzyme catalyzes O-glycan branch synthesis of the core 2 and core 4 type in mucins and controls expression of core 2 branched oligosaccharides and I antigens on the cell surface.