||Two families of transcription factors are important for executing the differentiation program in proliferating myoblasts to become contractile myotubes. These are the myocyte enhancer factors (MEF2s) and the myogenic regulatory factors (MRFs). Recent focus has been on the regulation of these factors by histone acetyltransferases (HATs) and histone deacetylases (HDACs). Specifically, MyoD (a MRF) is bound and acetylated by HAT such as p300 and P/CAF. Unlike wild-type MyoD that is sufficient to induce myogenesis when over-expressed, specific non-acetylatable MyoD mutants are impaired in this process. Similarly, Class I HDAC1, which can deacetylate acetylated MyoD in vitro, potently represses MyoD-dependent gene transcription and myogenesis. On the other hand, activities of MEF2 isoform C are notably augmented by HAT such as p300 and repressed by Class II HDACs but the underlying molecular mechanism is obscure. We propose that MEF2C, which contains over 15 lysine residues, is an ideal substrate of p300, and have successfully identified key acetylatable lysine residues both in vitro and in cells. More importantly, specific non-acetylatable MEF2 mutants are transcriptionally inactive and are impaired in enhancing myogenesis. On the other hand, while it is known that histone deacetylase four (HDAC4), a potent Class II repressor of myogenesis, inhibits MEF2-dependent gene expression by first docking to the MADSMEF domain of MEF2s, HDAC4(1-208), a region of HDAC4 devoid of any intrinsic deacetylase activity, can still intriguingly repress MEF2. We discovered that HDAC4(1-208) does not repress by disrupting the binding of MEF2 to its cognate DNA, but rather by recruiting additional HDAC via a region between residues 119-208. Interestingly, coexpression of HDAC4(1-208) (or the full-length HDAC4) and MEF2 alters the cellular distribution patterns of both proteins. Finally, the inhibitory effect of HDAC4 and HDAC4(1-208)/(119-208) on MEF2-driven transcription can be alleviated by MyoD, suggesting that MyoD might simply compete out the binding of HDAC4 to MEF2.