Most plasmid vectors used today contain two selectable markers — one is used to select transformants and the other to select recombinants (via insertional inactivation).
Although this enhances the ability to select recombinants, plasmid vectors have also been constructed to allow direct selection, whereby only transformed host cells harboring a vector containing foreign DNA are able to grow on the selective media, such as pBD214 (selection of recombinant plasmids in Bacillus subtilis) and pPW264 (cloning of the gene for α-amylase from Schwanniomyces occidentalis).