Spliceosome-mediated RNA trans-splicing (SMaRT) has been used previously to reprogram mutant endogenous CFTR and factor VIII mRNAs in human epithelial cell and tissue models and knockout mice, respectively. Those studies used 3 exon replacement (3ER); a process in which the distal portion of RNA is reprogrammed. Here, we also show that the 5 end of mRNA can be completely rewritten by 5ER. For proof-of-concept, and to test whether 5ER could generate functional CFTR, we generated a mutant minigene target containing CFTR exons 10-24 (⌬F508) and a mini-intron 10, and a pretrans-splicing molecule (targeted to intron 10) containing CFTR exons 1-10 (+F508), and tested these two constructs in 293T cells for anion efflux transport. Cells cotransfected with target and PTM showed a consistent increase in anion efflux, but there was no response in control cells that received PTM or target alone. Using a LacZ reporter system to accurately quantify trans-splicing efficiency, we tested several unique PTM designs. These studies provided two important findings as follows: (1) efficient trans-splicing can be achieved by binding the PTM to different locations in the target, and (2) relatively few changes in PTM design can have a profound impact on trans-splicing activity. Tethering the PTM close to the target 3 splice site (as opposed to the donor site) and inserting an intron in the PTM coding resulted in a 65-fold enhancement of LacZ activity. These studies demonstrate that (1) SMaRT can be used to reprogram the 5 end of mRNA, and (2) efficiency can be improved substantially.