RIBOZYMES can be targeted to cleave specific RNAs^1–8, which has led to much interest in their potential as gene inhibitors^3,9,10. Such fra/is-cleaving ribozymes join a growing list of agents that stop the flow of genetic information^11,12. Here we describe a different application of ribozymes for which they may be uniquely suited. By targeted trans-splicing, a ribozyme can replace a defective por-tion of RNA with a functional sequence. The self-splicing intron from Tetrahymena thermophila¹³ was previously shown to mediatetrans-splicing of oligonucleotides in vitro^14,15. As a model system for messenger RNA repair, this group I intron was re-engineered to regenerate the proper coding capacity of short, truncated lacZ transcripts. Trans-splicing was efficient in vitro and proceeded in Escherichia coli to generate translatable lacZ messages. Targeted frans-splicing represents a general means of altering the sequence of specified transcripts and may provide a new approach to the treatment of many genetic diseases.