A method of producing a transgenic plant or a transgenic plant cell which is capable of adding galactose residues in β1,4-linkage to N-linked glycans, comprising: (a) inserting a nucleic acid molecule into a plant or a plant cell, the nucleic acid molecule comprising a nucleotide sequence coding for a non-mammalian β1,4-galactosyltransferase selected from the group consisting of: (i) a polypeptide comprising an amino acid sequence at least 85% identical to a zebrafish β1,4-galactosyltransferase set forth as SEQ ID NO:14, (ii) a polypeptide comprising a first fragment consisting of an amino acid sequence of about 10-20 amino acids in length corresponding to the N-terminal amino acid sequence of a mammalian β1,4-galactosyltransferase and a second fragment that is the zebrafish β1,4-galactosyltransferase of SEQ ID NO:14, the first fragment being linked to the N-terminal of the second fragment and comprising the sequence [K/R]-X-[K/R] in the first 10 N-terminal amino acids, wherein [K/R] represents either a lysine or arginine residue and X representing any amino acid, and (iii) a polypeptide comprising an amino acid sequence that is otherwise identical to the zebrafish β1,4-galactosyltransferase of SEQ ID NO:14 except that the cytoplasmic-transmembrane-stem region of the zebrafish β1,4-galactosyltransferase is replaced by the CTS of a mammalian sialyltransferase or by the CTS of a plant Golgi-localized protein. (b) selecting a transgenic plant or transgenic plant cell that has taken up the nucleic acid molecule of (a); and (c) expressing the nucleic acid molecule, thereby producing a transgenic plant or transgenic plant cell capable of adding galactose residues in β1,4-linkage to N-linked glycans. (a) inserting a nucleic acid molecule into a plant or a plant cell, the nucleic acid molecule comprising a nucleotide sequence coding for a non-mammalian β1,4-galactosyltransferase selected from the group consisting of: (i) a polypeptide comprising an amino acid sequence at least 85% identical to a zebrafish β1,4-galactosyltransferase set forth as SEQ ID NO:14, (ii) a polypeptide comprising a first fragment consisting of an amino acid sequence of about 10-20 amino acids in length corresponding to the N-terminal amino acid sequence of a mammalian β1,4-galactosyltransferase and a second fragment that is the zebrafish β1,4-galactosyltransferase of SEQ ID NO:14, the first fragment being linked to the N-terminal of the second fragment and comprising the sequence [K/R]-X-[K/R] in the first 10 N-terminal amino acids, wherein [K/R] represents either a lysine or arginine residue and X representing any amino acid, and (iii) a polypeptide comprising an amino acid sequence that is otherwise identical to the zebrafish β1,4-galactosyltransferase of SEQ ID NO:14 except that the cytoplasmic-transmembrane-stem region of the zebrafish β1,4-galactosyltransferase is replaced by the CTS of a mammalian sialyltransferase or by the CTS of a plant Golgi-localized protein. (i) a polypeptide comprising an amino acid sequence at least 85% identical to a zebrafish β1,4-galactosyltransferase set forth as SEQ ID NO:14, (ii) a polypeptide comprising a first fragment consisting of an amino acid sequence of about 10-20 amino acids in length corresponding to the N-terminal amino acid sequence of a mammalian β1,4-galactosyltransferase and a second fragment that is the zebrafish β1,4-galactosyltransferase of SEQ ID NO:14, the first fragment being linked to the N-terminal of the second fragment and comprising the sequence [K/R]-X-[K/R] in the first 10 N-terminal amino acids, wherein [K/R] represents either a lysine or arginine residue and X representing any amino acid, and (iii) a polypeptide comprising an amino acid sequence that is otherwise identical to the zebrafish β1,4-galactosyltransferase of SEQ ID NO:14 except that the cytoplasmic-transmembrane-stem region of the zebrafish β1,4-galactosyltransferase is replaced by the CTS of a mammalian sialyltransferase or by the CTS of a plant Golgi-localized protein. (b) selecting a transgenic plant or transgenic plant cell that has taken up the nucleic acid molecule of (a); and (c) expressing the nucleic acid molecule, thereby producing a transgenic plant or transgenic plant cell capable of adding galactose residues in β1,4-linkage to N-linked glycans.