Electrophoresis of plasmid DNA (pDNA) fragments prepared by sonication. DNA mobility was analysed by agarose-gel electrophoresis (3·5% gel). Lane 1, Φ-HaeIII marker; lane 2, control pCMV-Luc; lanes 3–6, pCMV-Luc fragments produced by sonication for 30, 60, 90 and 120 min, respectively.