Cover photograph (Copyright © 2000, American Society for Microbiology. All rights reserved.): Molecular structure of the BTB-POZ dimerization domain of the promyelocytic leukemia zinc finger (PLZF) protein. (Top) Ribbon representation of the dimer with monomers shown in blue and red. The monomers are held together by an extensive interface stabilized by conserved hydrophobic interactions. A highly conserved pocket can be seen at the very top of the dimer interface. (Bottom) Space filling-charge mapping representation of the PLZF BTB-POZ dimer with acidic and basic regions shown in red and blue, respectively. Mutants that destabilize the charged pocket are deficient in dimerization and repression, but mutants that neutralize the pocket charge allow dimerization without repressing transcription. Thus, the pocket is involved in mediating BTB-POZ-dependent transcriptional repression. (See related article on p. 6550.)