The NF-?B transcription factor is part of a signaling pathway involved in the regulation of cell growth, differentiation and cellular survival. NF-?B activity is often found deregulated in diseases associated with chronic inflammation. Accumulating evidence suggest that NF-?B also functions in skeletal muscle disorders, and its involvement and mechanisms of action in several of these disorders are now beginning to defined. Our approach to furthering our insight in how NF-?B participates in muscle disease is to ascertain on a more basic level how this signaling pathway functions in regulating skeletal muscle differentiation. Results revealed that NF-?B participates in myogenesis through two signaling pathways, referred to as the classical and alternative. Whereas the classical is constitutively active in proliferating myoblasts and inhibits differentiation through multiple mechanisms, alternative signaling is induced during myogenesis, and functions not in myotube formation, but rather in promoting mitochondrial biogenesis and maintaining myotube homeostasis. The goal of this current application is to perform a more in depth analysis of the alternative pathway by elucidating the mechanism and relevance in regulating mitochondrial biogenesis and oxidative metabolism in differentiating muscle cells. Towards this goal we seek to perform the following three specific aims: 1) Determine the molecular mechanism by which alternative signaling regulates mitochondrial biogenesis in differentiating muscle cells; 2) Determine whether regulation of mitochondrial biogenesis by alternative NF-?B signaling is relevant in vivo; and 3) Elucidate the mechanism by which the alternative pathway becomes activated during myogenesis. Since mitochondrial dysfunction is often associated with muscle disorders, broadening our understanding of alternative NF-?B signaling in myogenesis may shed additional insight in how NF-?B participates in skeletal muscle disease.