The invention provides a nucleic acid or expression vector for use in delivering a nucleic acid or expression vector to a target organ or tissue in an animal comprising: transfecting at least one cell of a first tissue in said animal with the nucleic acid or expression vector, wherein said transfection results in said nucleic acid or expression vector being delivered to at least one cell in said target tissue or organ, wherein said target organ or tissue is distal from the site of said transfected first cell.
