A recombinant host cell comprising a vector comprising each of the following elements sequentially arranged to form a modular open reading frame, said vector comprising: a) a first restriction site encoding a first group of amino acids, the first comprising amino acid group at least two amino acids, b) a first open reading frame encoding a polypeptide of interest, in which the coding sequence of the polypeptide of interest is in frame with the coding sequence of at least two amino acids of the first site of restriction, c) a second restriction site encoding a second group of amino acids, the second group of amino acids comprising at least two amino acids, in which the coding sequence of the second group of amino acids of the second restriction site is in frame with the coding sequence of the polypeptide of interest of the first open reading frame or, and wherein the polynucleotide sequence of the second restriction site is not the same as the polynucleotide sequence of the first restriction site, d) a spacer polynucleotide sequence, encoding the spacer polynucleotide sequence for at least three spacer amino acids, and in the that the coding sequence for the spacer sequence is in frame with the polynucleotide sequence of the second restriction site, and e) a third restriction site encoding a third group of amino acids, the third group of amino acids comprising at least two amino acids, in the that the coding sequence of the third group of amino acids of the third restriction site is in frame with the coding sequence of the at least three spacer amino acids and in which the polynucleotide sequence of the third restriction site is not the same as the polynucleotide sequence of the first restriction site and the third site of re striction is not the same as the polynucleotide sequence of the second restriction site； 3 0 in which the elements (a) to (e) are linked together to form the modular open reading frame, in which the first restriction site is susceptible to a res
