The current invention comprises a nucleic acid encoding the amino acid sequence of the C-terminal part of the CH3-domain of an immunoglobulin of the class IgA or IgG, or the C-terminal part of the CH4-domain of an immunoglobulin of the class IgE or IgM, wherein the glycine-lysine-dipeptide comprised in the amino acid sequence of the C-terminal part of the CH3- or CH4-domain is encoded by the nucleic acid ggaaaa, or the nucleic acid ggcaaa, or the nucleic acid gggaaa, or the nucleic acid gggaag, or the nucleic acid ggcaag, or the nucleic acid ggaaag.