The p53 protein is a tetrameric transcription factor that plays a central role in the prevention of neoplastic transformation. Oligomerization appears to be essential for the tumor suppressing activity of p53 because oligomerization-deficient p53 mutants cannot suppress the growth of carcinoma cell lines. The crystal structure of the tetramerization domain of p53 (residues 325 to 356) was determined at 1.7 angstrom resolution and refined to a crystallographic R factor of 19.2 percent. The monomer, which consists of a beta strand and an alpha helix, associates with a second monomer across an antiparallel beta sheet and an antiparallel helix-helix interface to form a dimer. Two of these dimers associate across a second and distinct parallel helix-helix interface to form the tetramer.