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Although adeno-associated virus (AAV)-mediated gene therapy has been hindered by the small viral packaging capacity of the vector, trans-splicing AAV vectors are able to package twice the size of the vector genome. Unfortunately, the efficiency of current trans-splicing vectors is very low. Here we show that rational design of the gene splitting site has a profound influence on trans-splicing vector-mediated gene expression. Using mRNA accumulation as a guide, we generated a set of efficient trans-splicing vectors and achieved widespread expression of the 6-kb ΔH2-R19 mini-dystrophin gene in skeletal muscle of mdx mice, a model for Duchenne muscular dystrophy. The dystrophic phenotype was ameliorated in both adult and aged mice. This demonstrates the use of trans-splicing vectors to efficiently express a large therapeutic structural protein. This strategy should be applicable to other large therapeutic …