Minicircle DNA production in vivo. Minicircle DNA was produced using producer plasmid pFIXluc. Electrophoresis was performed in a 0.7% agarose gel in the presence of ethidium bromide. (A) Analysis of the supercoiled DNA extracted from the recA- strain HB101Cre, the recA+ strain MM219Cre and the recA+ strain TB1Cre, all harbouring pFIXluc, before and after induction of the Cre gene expression with L-arabinose. Intramolecular Cre recombination in the minicircle producer plasmid pFIXluc (6456 bp) gave rise, in particular, to the monomeric minicircle mFIXluc (3089 bp) and the monomeric miniplasmid (3367 bp). (B) Agarose gel analysis of the purified minicircle mFIXluc isolated from the recA- strain HB101Cre pFIXluc and the recA+ strain MM219Cre pFIXluc.