of the extracellular matrix and the sarcolemma, electron microscopy was performed on non-Tg and phenotypic SSPN-Tg muscle. Electron micrographs of EDL muscles reveal that SSPN-Tg muscle possesses normal sarcomere structures (Fig.  9A). Defects in the sarcolemma of SSPN-Tg muscle were not observed, supporting **data** from the Evans Blue dye and creatine kinase assays (Fig. 3). However, the distances between sarcolemma of adjacent myofibers appeared to be greater in the SSPN-Tg muscle compared with non-Tg controls (white arrows, Fig. 9A,B). Photomicrographs of the sarcolemma taken at higher magnification show that the extracellular matrix in SSPN-Tg muscle is characterized by randomly placed, unorganized dense clumps (black arrows, Fig. 9B). By contrast, the extracellular matrix from non-Tg muscle appeared to be a tight, linear structure that was closely aligned in a parallel fashion with the sarcolemma (Fig. 9B). The disorganization of the basement membrane in SSPN-Tg muscle may account for the larger spacing between neighboring myofibers. Perturbation of the extracellular matrix represents a likely pathological mechanism for the severe and lethal dystrophic phenotype observed in the SSPN-Tg mice, as proposed in our model (Fig. 10).