Because nNOS is displaced in dystrophin-deficient skeletal muscle cells, nNOS activity levels are considerably lower in DMD patients and mdx mice, although nNOS deficiency is not due solely to lack of dystrophin in localizing the protein to the membrane as mRNA level of nNOS was also reduced in muscle biopsy of DMD patients and mdx mouse muscle [115]. This evidence concerns the gene NOS1 and Duchenne muscular dystrophy.