The application discloses a method for producing an oligosaccharide of at least four monosaccharide units, advantageously an HMO, particularly an HMO of only four monosaccharide units, said method comprising a step of: culturing, in a culture medium containing a fucosylated, sialylated or N-acetyl-glucosaminylated lactose trisaccharide as acceptor, a genetically modified cell having a recombinant gene that encodes an enzyme capable of modifying said acceptor or one of the necessary intermediates in the biosynthetic pathway of the oligosaccharide of at least four monosaccharide units, advantageously an HMO, particularly an HMO of only four monosaccharide units, from said acceptor.