A retroviral vector comprising a first retroviral envelope protein and at least one modified retroviral envelope protein, wherein the first retroviral envelope protein includes a surface protein comprising (i) a receptor binding region; (ii) a hypervariable polyproline region; and (iii) a body portion, and the modified retroviral envelope protein, prior to modification, includes a surface protein which includes (i) a receptor binding region; (ii) a hypervariable polyproline region; and (iii) a body portion, characterized in that the modified retroviral envelope protein has been modified such that at least 90% of the amino acid residues of the receptor binding region of the surface protein of the modified retroviral envelope protein have been removed and replaced with a non-retroviral protein or peptide.