Because of the lethal effects of the CcdB protein, all vectors containing the ccdB gene must be propagated in an E. coli strain that is resistant to CcdB effects. We recommend using the DB3.1™ E. coli strain which contains a gyrase mutation (gyrA462) that renders it resistant to the CcdB effects (Bernard and Couturier, 1992; Bernard et al., 1993; Miki et al., 1992).