Abstract: The invention relates to a process for the removal of selectable marker gene sequences, in particular antibiotic gene sequences, from nucleic acid molecules. The invention further relates to the application of this process in the unlabelled integration and deletion of chromosomal genes and in controlling gene expression.

Abstract: 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.