text: All HMOs derive from lactose, which can be decorated by four monosaccharides (N-acetyl-D-glucosamine, D-galactose, sialic acid and/or L-fucose) to form an oligosaccharide. The HMO variability in human mothers depend on two specific enzymes, the α1-2-fucosyltransferase (FUT2) and the α1-3/4-fucosyltransferase (FUT3). The milk of mothers with inactivated FUT2 enzyme do not contain α1-2-fucosylated HMOs, and likewise with inactivated FUT3 enzyme there can be almost no α1-4-fucasylated HMOs found. Typically 20% of the global population of mothers do not have active FUT2 enzyme, but still have an active FUT3 enzyme, whereas 1% of mothers express neither FUT2 nor FUT3 enzymes.