Abstract: Gal4 is a Zn2Cys6 binuclear cluster containing transcription factor that binds DNA as a homodimer and can activate transcription by interacting with the mutant Gal11P protein. Although structures have been reported of the Gal4 dimerization domain and the binuclear cluster domain bound to DNA as a dimer, the structure of the "complete" Gal4 dimer bound to DNA has not previously been described. Here we report the structure of a complete Gal4 dimer bound to DNA and additional biochemical studies to address the molecular basis for Gal4 dimerization in DNA binding. We find that Gal4 dimerization on DNA is mediated by an intertwined helical bundle that deviates significantly from the solution NMR structure of the free dimerization domain. Associated biochemical studies show that the dimerization domain of Gal4 is important for DNA binding and protein thermostability. We also map the interaction surface of the Gal4 dimerization domain with Gal11P.