Strains that contain an F plasmid, such as TOP10F', are not recommended for transformation and selection of recombinant clones in any TOPO vector containing the ccdB gene. While the F plasmid does encode the CcdA protein, which acts as an inhibitor of the CcdB gyrase-toxin protein, the half-life of the CcdA protein is shorter than that of the CcdB protein. Overexpression of the CcdB protein causes cell death when its action is not prevented by sufficient CcdA.