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.
