The invention relates to a system for stable maintenance of a plasmid, to
host cells for use in this system and to methods of using the system to
obtain a plasmid useful in medical applications. In particular, the
invention provides transformed host cell containing: i) a chromosomal
gene which inhibits cell growth; and ii) a plasmid encoding an antisense
sequence, wherein the antisense sequence encoded by the plasmid inhibits
the action of the chromosomal gene, thereby permitting cell growth and a
method for stable maintenance of a plasmid in a host cell in vivo.