The present invention relates to a nucleic acid molecule comprising a low repetitive nucleotide sequence encoding a proline/alanine-rich amino acid repeat sequence. The encoded polypeptide comprises a repetitive amino acid sequence that forms a random coil. The nucleic acid molecule comprising said low repetitive nucleotide sequences can further comprise a nucleotide sequence encoding a biologically or pharmacologically active protein. Further, the present invention provides for selection means and methods to identify said nucleic acid molecule comprising said low repetitive nucleotide sequence. The present invention also relates to a method for preparing said nucleic acid molecules. Also provided herein are methods for preparing the encoded polypeptide or drug conjugates with the encoded polypeptide using the herein provided nucleic acid molecules. The drug conjugate may comprise a biologically or pharmacologically active protein or a small molecule drug. Also provided herein are vectors and hosts comprising such nucleic acid molecules.
