Dorer "et al"., in 2010, showed that ciprofloxacin, which interacts with DNA gyrase and causes production of double-strand breaks, induces expression of competence genes in "Helicobacter pylori", leading to increased transformation. In 2011 studies of "Legionella pneumophila", Charpentier "et al". tested 64 toxic molecules to determine which ones induce competence. Only six of these molecules, all DNA damaging agents, strongly induced competence. These molecules were norfloxacin, ofloxacin and nalidixic acid (inhibitors of DNA gyrase that produce double strand breaks), mitomycin C (which produces inter-strand cross-links), bicyclomycin (causes single- and double-strand breaks), and hydroxyurea (causes oxidation of DNA bases). Charpentier "et al". also showed that UV irradiation induces competence in "L. pneumophila" and further suggested that competence for transformation evolved as a response to DNA damage.