Ribozymes (catalytic RNAs) can be made to specifically cleave target RNAs that are involved in disease conditions and therefore have potential as therapeutic agents. Gene Shears Pty Ltd is developing hammerhead ribozyme technology for therapy against HIV infection, targeting either the tat gene or the RNA packaging sequence (Psi) of HIV. These ribozymes have been expressed from constructs that were introduced into hematopoietic cells in culture, thereby protecting the cells against viral infection. Two phase I clinical trials are underway to test the safety and feasibility of the approach with the anti-tat ribozyme in human subjects.