Galactoside 2‑L‑fucosyltransferase‑2 and ‑3 are enzymes for the synthesis of fucosylated human milk oligosaccharide isoforms. Fucosyltransferase‑2 adds fucose with α1‑2 glycosidic bonds to neutral-non-fucosylated human milk oligosaccharides (Kumazaki and Yoshida 1984) and thus also generates 2'‑fucosyllactose. Fucosyltransferase‑3 adds fucose with α1‑3 or α1‑4 glycosidic bonds (Xu et al. 1996) and thus generates besides others 3'‑fucosyllactose. Both these HMO isoforms are rather common and present in noteworthy concentrations in many milk samples (Ayechu-Muruzabal et al. 2018; Kunz et al. 2017; Thurl et al. 2010).

Fucosyltransferase‑2 is encoded by the secretor gene FUT2 whereas fucosyltransferase 3 is encoded by the Lewis blood group gene FUT3. Both genes can be active or inactive and their activity not only defines the presence of certain HMO isoforms, but also their concentration. For example, in milks were one or both enzymes are absent, neutral, non-fucosylated HMO isoforms take precedence because they are not changed into a different isoform by the addition of fucose(milk groups, lacto‑N‑tetraose (Kunz et al. 2017)).

Alternative names for the fucosyltransferase‑2 gene are SE; Se2; sej; SEC2; or B12QTL1. Alternatives for the enzyme are GDP-L-fucose:beta-D-galactoside, 2-alpha-L-fucosyltransferase 2, alpha(1,2) fucosyltransferase, alpha(1,2)FT 2, alpha(1,2)FT2, fucosyltransferase 2 (secretor status included), galactoside 2-L-fucosyltransferase, secretor blood group alpha-2-fucosyltransferase, and secretor factor https://www.ncbi.nlm.nih.gov/gene/2524.

Alternative names for the fucosyltransferase-3 gene are LE; Les; FT3B; CD174; or FucT-III; Alternatives for the enzyme are galactoside 3(4)-L-fucosyltransferase, Lewis FT, alpha-(1,3/1,4)-fucosyltransferase, blood group Lewis alpha-4-fucosyltransferase, fucosyltransferase 3 (galactoside 3(4)-L-fucosyltransferase, Lewis blood group), and fucosyltransferase III https://www.ncbi.nlm.nih.gov/gene/2525.