Two such genes,
important for determining the HMO profile a mother produces,
are the Secretor and Lewis blood group genes.
The Secretor gene encodes for the enzyme α[1,2]-fucosyltransferase (FUT2),
responsible for linking fucose in a α1-2 linkage to elongate the HMO chain.
The enzyme FUT3 is encoded for by the Lewis blood group gene;
this enzymecatalyses the reaction between fucose in a α1-3/4 linkage,
creating further fucosylated oligosaccharides.
As a result of the different expressions of these enzymes,
there are four main phenotypes in relation to HMO profile: