An oligonucleotide including at least one concatenation coding for a polypeptide of formula (P--K).sub.n, wherein n is an integer of at least 2, P is a proline amino acid residue, K is a lysine amino acid residue, and the sign "--" is a bond, particularly a peptide bond, between the two amino acid residues. The n (P--K) units are also bound together by such bonds, e.g. peptide bonds.