Temperature-sensitive yeast mutants have been isolated that block each of the enzymatic steps in the synthesis of the dolichol-oligosaccharide precursor for N-linked glycosylation. Propose an explanation for why mutations that block synthesis of the intermediate with the structure dolichol-PP-(GlcNAc)₂Man₅ completely prevent addition of N-linked oligosaccharide chains to secretory proteins, whereas mutations that block conversion of this intermediate into the completed precursor––dolichol-PP-(GlcNAc)₂Man₉Glc₃––allow the addition of N-linked oligosaccharide chains to secretory glycoproteins.