DMD and Duchenne muscular dystrophy: Double mutant mice with the mdx background have been created in the attempt to approach the severe phenotype observed in DMD patients, such as the double knockout mdx:utrn-/-, with absence of both dystrophin and utrophin [13, 14]; the mdx/mTR mouse, with impaired telomerase activity [15]; and the Dmdmdx-Largemyd mouse (hereafter called mdx/Largemyd), recently generated in our laboratory by crossing mdx and Largemyd murine lineages [16].