Antisense and ribozymes have a relatively short yet successful history as research tools in gene expression studies, and thus are considered as having high potential reagents in treating viral infections and cancer. This laboratory companion provides detailed information on the potential, advantages and limitations of this methodology. It critically discusses potential pitfalls, presents strategies for choosing targets and delivery systems, so as to allow the selection of the optimum methodology for achieving fast and reliable experimental success with any human or other biological system. For.