Over the past two decades, the structure and mechanism of catalytic RNA have been extensively studied; now ribozymes are understood well enough to turn them into useful tools. After we have demonstrated the twin ribozyme mediated insertion of additional nucleotides into a predefined position of a suitable substrate RNA, we here show that a similar type of twin ribozyme is also capable of mediating the opposite reaction: the site-specific removal of nucleotides. In particular, we have designed a twin ribozyme that supports the deletion of four uridine residues from a given RNA substrate. This reaction is a kind of RNA recombination that in the specific context of gene therapy mimics, at the level of RNA, the correction of insertion mutations. As a result of the twin ribozyme driven reaction, 17% of substrate are converted into the four nucleotides shorter product RNA. (C) 2007 Elsevier Inc. All rights reserved.