The simplest routes for glycosyltransferase-catalyzed enzymatic synthesis of complex mammalian glycans with in situ generation of sugar nucleotides from (A) a monosaccharide other than sialic acid or (B) sialic acid (Sia). Enzyme abbreviations: GlyK, glycokinase; NucT, nucleotidyltransferase; GlyT, glycosyltransferase; PpA, inorganic pyrophosphatase; aldolase, sialic acid aldolase, CSS, CMP-sialic acid synthetase; SiaT, sialyltransferase.