The invention relates to nucleic acid molecules encoding preproproteins having after maturation the biological activity of the mistletoe lectin dimer, to vectors comprising these nucleic acid molecules, to hosts transformed with said vectors and to polypeptides and/or polypeptide dimers which are encoded by these nucleic acid molecules. The polypeptides and/or polypeptide dimers of the invention are widely therapeutically applicable. Thus, the present invention further relates to immunotoxins as well as to pharmaceutical compositions that contain the polypeptides and/or the polypeptide dimers of the invention. Additionally, the invention relates to diagnostic compositions comprising the nucleic acid molecules of the invention, the polypeptides and/or the polypeptide dimers of the invention and/or primers which hybridize specifically to the nucleic acid molecules of the invention. Finally, the invention relates to plant protective agents comprising the polypeptides of the invention and/or the polypeptide dimers of the invention.