This invention provides in vivo methods of maintaining a plasmid within a host cell, which is in turn within a recipient organism. In particular it relates to methods of maintaining a plasmid containing a gene useful in therapy, especially vaccination. The invention is based on modified host cells, suitable for in vivo therapeutic use, that may be used to express a therapeutically useful plasmid-borne gene, using a plasmid maintenance system that does not require the use of a plasmid-borne dominant selectable marker, but rather utilises a system of repressor titration. Methods of selecting host cells comprising the plasmid and host cells for use in these methods are also provided.