Abstract: The stable maintenance of a replicon in a population of growing cells is ensured by providing the replicon with a sequence which encodes a product capable of killing the cell harboring the replicon or the progeny of the cell (or encodes a precursor for the product) and a sequence encoding an antagonist for the killing product (or a precursor for the antagonist). The antagonist is one which suppresses the killing product (or a precursor for the killing product) in cells harboring the replicon, whereas the antagonist activity decays when the replicon is lost from the cell so that the antagonist (or its precursor) is no longer continuously expressed. This means that the killing product (or its precursor) present in the now replicon-free cell is no longer suppressed by the antagonist, resulting in cell death.Cells containing the thus stabilized replicon may be grown on a large scale without any significant loss of the replicon from the cell population even when no selection pressure is applied.