Le gene or FUT3 https://www.ncbi.nlm.nih.gov/gene/2525;
Together with the secretor gene FUT2, the Lewis blood group gene regulates presence and concentrations of fucosylated human milk oligosaccharides. Both genes encode for fucosyltransferase enzymes that add fucose to an oligosaccharide in different ways and are well known to control blood group antigens (Dotz and Wuhrer 2016).
Women with an active Lewis blood group gene express the fucosyltransferase-3 variant, which marks them "Lewis‑" or "Le‑positive". If the Le gene is inactive, fucosyltransferase‑3 is absent and individuals are first described as "Le‑negative". The four combinations of secretor/non‑secretor status and Le-positive/negative were first described as four global milk groups in 1997 (Thurl et al. 1997).