The present invention provides a retroviral or lentiviral vector having a viral envelope which comprises: (i) a mitogenic T cell activating transmembrane protein which comprises a mitogenic domain and a transmembrane domain and/or (ii) a cytokine based T cell activating transmembrane protein which comprises a cytokine domain and a transmembrane domain wherein the mitogenic or cytokine based T cell activating transmembrane protein is not part of a viral envelope glycoprotein. When cells such as T cells of Natural Killer cells are transduced by such a viral vector they are simultaneously activated by the mitogenic T cell activating transmembrane protein and/or the cytokine based T cell activating transmembrane protein.
