NOS1 and Duchenne muscular dystrophy: Utrophin has a similar structure and function to dystrophin, but lacks two spectrin-like repeats, as well as the critical nNOS-binding site.21,22 It is primarily expressed in skeletal muscle during fetal development in both mice and humans, and is down-regulated before birth in normal muscle.23 In the absence of dystrophin, utrophin is often up-regulated, but due to the functional differences in the protein, it does not fully compensate for dystrophin in human DMD.24