The N -glycan encoding system of GlycoForm and Glycologue. (a) The trimannosyl core structure, Man3, with the numeric value 300000000. (b) Extension levels of a branch attached to one of the exposed mannose residues of the core structure. The value of "x" is either 6 or 2 when a branch occurs at either of these two positions on the "upper" (α-1,6) mannose; "x" takes the values 4 or 2 at those positions on the "lower" (α-1,3) mannose. The extension level of a branch is encoded as a single digit from 0-6. Extension levels 3 and 6 signify a N-acetylneuraminate (sialic acid) residue α-2,3-linked to galactose.