10,000 proteins identified in C2C12 myogenesis revealed a novel subset of 1,239 myogenically-regulated proteins. This set of regulatory proteins clustered into five distinct longitudinal expression trends which significantly overlap those obtained in similar analyses performed in human myocytes. A longitudinal analysis of myogenesis in the presence of tamoxifen, when contrasted with a similar analysis in untreated myogenesis finds that while the vast majority of myogenically-regulated proteins were unaffected by tamoxifen treatment, specific pathways and networks are affected. We document a specific functional enrichment for adiponectin-signaling, whereby a set of 198 proteins were differentially expressed relative to controls at one or more stages of myogenesis, the majority of which were involved in steroid biosynthesis, lipid storage and/or metal ion homeostasis. Interestingly, the only protein that was differentially expressed in the tamoxifen-treated cells at every stage of myogenesis was metallothionein-1 (MT1). Elevated levels of MT1 have been correlated with tamoxifen resistance and increased patient mortality and relapse in breast cancer, as well as with cachexia and muscle atrophy in rodent models. Increased MT1 expression levels may contribute to the musculoskeletal effects reported by patients undergoing tamoxifen treatment. Finally, we present a powerful, self-validating pipeline for analyzing the total proteomic response to in vitro treatment across every stage of muscle cells development which can be easily adapted to study the effects of other drugs on myogenesis. ]]>