Human milk oligosaccharide isoforms can be characterised to three different human milk oligosaccharide categories (HMO categories), on base of their structure. (1) Fucosylated HMO: Fucosylated HMO are linked with at least one L-fucose residue. Representatives are 2'‑fucosyllactose and 3'‑fucosyllactose. (2) Sialylated or acidic HMO are linked with at least one sialic acid residue. Representatives are 3'‑sialyllactose or 6'‑sialyllactose. (3) Neutral, non-fucosylated HMO are linked with at least one galactose, N‑acetyllactosamine, or lacto‑N‑biose unit. They are neither linked with fucose nor sialic acid. Representatives are lacto‑N‑neotetraose (Kunz et al. 2000) or 3'‑galactosyllactose (Ayechu-Muruzabal et al. 2018).