The Tetrahymena group I intron ribozyme is an RNA molecule of approx 400 bases that is capable of base-specific RNA trans-splicing. These properties can be applied to repair mutations at the RNA level, and therefore can restore normal cellular functions in diseased cells. The purpose of this chapter is to present the methodology that can be used to repair mutations responsible for myotonic dystrophy. The design and construction of group I intron ribozymes and their transfection in myotonic dystrophy cells will be among the sections described in this chapter.