When cultured in low serum-containing growth medium, the mouse C 2 C 12 cells exit cell cycle and undergo a welldefined program of differentiation that culminates in the formation of myosin heavy chain-positive bona fide multinucleated muscle cells. To gain an understanding into this process, we compared total, membrane-and nuclear-enriched proteins, and phospho-proteins from the proliferating C 2 C 12 cells and the fully differentiated myotubes by the combined methods of two-dimensional PAGE, quantitative PDQuest image analysis, and MS. Quantification of more than 2,000 proteins from C 2 C 12 myoblasts and myotubes revealed that a vast majority of the abundant proteins appear to be relegated to the essential, housekeeping and structural functions, and their steady state levels remain relatively constant. In contrast, 75 proteins were highly regulated during the phenotypic conversion of rapidly dividing C 2 C 12 myoblasts into fully differentiated, multi-nucleated, post-mitotic myotubes. We found that differential accumulation of 26 phosphoproteins also occurred during conversion of C 2 C 12 myoblasts into myotubes. We identified the differentially expressed proteins by MALDI-TOF-MS and LC-ESIquadrupole ion trap MS/MS. We demonstrate that more than 100 proteins, some shown to be associated with muscle differentiation for the first time, that regulate interand intracellular signaling, cell shape, proliferation, apoptosis, and gene expression impinge on the mechanism of skeletal muscle differentiation. Molecular & Cellular