The stepwise addition of carbohydrate or carbohydrate derivative residues to the initially added O-linked residue (usually GalNAc) to form the core 1 O-glycan structure, Gal-beta-(1->3)-GalNAc. Core 1 occurs on most glycoproteins, and is often substituted with sialic acid residues or with various other sugars.
core 1 O-glycan biosynthesis