HMO consists of five structural units: an acid monosaccharide, sialic acidor N-acetylneuraminic acid; three monosaccharides, namely D-glucose (Glc), L-fucose (Fuc), and D-galactose (Gal); and an amino sugar, N-acetylglucosamine (GlcNAc). The reducing end structures of these units all contain a lactose moiety. HMOs can be divided into three categories: fucosylated oligosaccharides, sialylated oligosaccharides, and core oligosaccharides.