Members of the genus Clostridium are of both medical and industrial importance. The molecular tools necessary to study and exploit their wide ranging physiological diversity through directed mutational analysis have until recently been lacking. The situation was transformed in the mid-2000s with the specific adaptation of intron re-targeting technology to the genus, through the development of the ClosTron. By making a handful of nucleotide changes to the group II intron encoding region, the intron can be directed to insert into almost any region within the genome. Through the use of a retrotransposition-activated marker (RAM), based on the ermB gene, successful insertion is selected on the basis of acquisition of resistance to erythromycin. The re-targeted region is designed using an online re-targeting algorithm (www.clostron.com), and then an order is placed with DNA2.0 for both the synthesis of the re-targeted region and its custom cloning into the ClosTron vector. Re-targeted ClosTrons are delivered ready for use in 10-14 days, allowing mutants to be isolated 5-7 days after receipt. Its availability has revolutionized clostridial molecular biology.