A retroviral delivery system capable of transducing a target site is described. The retroviral delivery system comprises a first nucleotide sequence coding for at least a part of an envelope protein; a second nucleotide sequence coding for at least a part of an envelope protein and one or more other nucleotide sequences derivable from a retrovirus that ensure transduction of the target site by the retroviral delivery system; wherein the first and second nucleotide sequences are heterologous with respect to at least one of the other nucleotide sequences; and wherein the first nucleotide sequence codes for at least a part of an influenza HA protein or a mutant, variant, derivative or fragment thereof that is capable of recognising the target site. Preferably said second protein is an influenza M2 protein.