The ability of peptide nucleic acids and hammerhead ribozymes, which target different subunits of human telomerase, to efficiently inhibit the enzyme's catalytic activity has been clearly demonstrated in several in vitro studies carried out in human immortalized and cancer cell lines. However, the actual efficacy of these molecules still needs to be validated in in vivo human tumor models, and such validation appears to be largely dependent on the development of reliable systems for their intracellular delivery.