Molecular architecture of the mSin3A corepressor complex was initially discovered as a corepressor for Mad1:Max heterodimers, and it was the first corepressor identified in mammalian systems. Our lab was among the first to show that mSin3A is present in cells as a large multiprotein complex and depends on associated histone deacetylases for its full corepressor function. This was the first description of how histone deacetylases are targeted to promoter regions. We have gone on to identify a number of proteins, such as MRG15, Pf1 and TLE1, which interact with the mSin3A complex in substoichiometric amounts. We have also identified SAP130, SAP180, and mSDS3 as core stoichiometric members of the mSin3A complex. We are now determining how each of these mSin3A-associated factors contribute to the assembly, targeting, and regulation of the mSin3A complex.