The ability to express functional microdystrophins from coding sequences small enough to be carried by AAV vectors is based on (1) studies of deletions that removed various domains from dystrophin and their effects on functionality of the protein, as discussed under Biological Plausibility, and (2) the knowledge gained from natural deletions occurring in BMD patients.63 The critical functional elements of dystrophin are described below (Fig. 1):. This evidence concerns the gene DMD and Becker muscular dystrophy.