The present invention provides a nucleic acid construct comprising the following structure: A-X—B in which A is nucleic acid sequence encoding a first polypeptide which comprises a first signal peptide B is nucleic acid sequence encoding a second polypeptide which comprises a second signal peptide and X is a nucleic acid sequence which encodes a cleavage site, wherein the first signal peptide or the second signal peptide comprises one or more mutation(s) such that it has fewer hydrophobic amino acids.
