In the context of human milk oligosaccharides (HMO), four global milk group have been identified. The grouping is based on presence of certain, mostly fucosylated human milk oligosaccharide isoforms. These differences are genetically controlled by the secretor‑ and Lewis blood group genes. The combination of active/ inactive Lewis blood group gene with secretor/non-secretor status, defines expression of those enzymes that add fucose to an oligosaccharide (fucosyltransferases). This changes the composition of milk (Thurl et al. 1997).