Methods and Protocols METHODS IN MOLECULAR BIOLOGY Since the first reports of the fascinating phenomenon of catalytically active RNAs in the 1980s, ribozyme research has become a major field in the molecular and biological sciences. Following the initial, ground-breaking discoveries, in the 1990s phosphodiester-cleaving ribozymes have been studied intensively both as basic tools as well as for therapeutic applications in order to suppress target gene expression. The past decade has then seen a variety of fundamental insights into ribozymes which have profoundly changed our view on RNA catalysis: The increasing biochemical and structural knowledge of ribosomal protein synthesis revealed that the fundamental peptidyl-transfer reaction is catalyzed by rRNA.