The present invention relates to a hybrid protein comprising an active site of mammalian UDP-N-acetylglucosamine: ²-D mannoside ²(1,4)-N-acetylglucosaminyltransferase (GnTIII) and a transmembrane region (Tm) of a protein that resides in the endoplasmic reticulum or Golgi apparatus of a eukaryotic cell.