There is provided a method for inserting a nucleic acid sequence that encodes a foreign peptide into a poxvirus genome, said method comprising: identifying in the poxvirus genome a poxvirus open reading frame wherein said open reading frame is characterized by an initial ATG start codon and wherein expression of said open reading frame is driven by an operably-linked poxvirus promoter located upstream of the open reading frame and wherein expression of said open reading frame provides a peptide that is non-essential to viability of the poxvirus; and inserting the nucleic acid sequence that encodes the foreign peptide at a position downstream of the poxvirus promoter; wherein following said insertion, (i) the nucleic acid that encodes the foreign peptide is operably-linked to the poxvirus promoter and expression of said nucleic acid is driven by said poxvirus promoter; and (ii) translation of the foreign peptide is initiated at an ATG start codon located at the same position as the ATG start codon of the poxvirus open reading frame. Also provided are a poxvirus vector and corresponding uses of the poxvirus vector in medicine.