zinc-finger domains to modulate specific target areas of the genome. In both designer and natural zinc-finger motifs, the protein consists of two β-sheets and one α-helix. Two histidine residues on the α-helix and two cysteine residues on the β-sheets are bonded to a zinc atom, which serves to stabilize the protein domain as a whole. This stabilization particularly benefits the α-helix in its function as the DNA-recognition and -binding domain. Transcription factor TFIIIA is an example of a naturally-occurring protein with zinc-finger motifs. How they work Zinc-finger motifs bind into the major groove of helical DNA, where the amino acid residue sequence on the α-helix gives the motif its target sequence specificity. The domain binds to a seven-nucleotide sequence of DNA (positions 1 through 6 on the primary strand of DNA, plus positions 0 and 3 on the complementary strand), thereby ensuring that the protein motif is highly selective of its target. In engineering a designer zinc-finger protein, researchers can utilize techniques such as site-directed mutagenesis followed by randomized trials for binding capacity, or the in vitro recombination of motifs with known target specificity to produce a library of sequence-specific final proteins. Effects and impacts on gene modulation Designer zinc-finger proteins can modulate genome expression in a number of ways. Ultimately, two factors are primarily responsible for the end result on expression: whether the targeted sequence is a regulatory region or a coding region of DNA, and whether and what types of effector domains are bound to the zinc-finger domain. If the target sequence for an engineered designer protein is a regulatory domain - e.g., a promoter or a repressor of replication - the binding site for naturally-occurring transcription factors will be obscured, leading to a corresponding decrease or increase, respectively, in transcription for the associated gene. Similarly, if the target sequence is an exon, the designer zinc-finger will obscure the sequence from RNA polymerase transcription complexes, resulting in a truncated or otherwise nonfunctional gene product. Effector domains bound to the zinc-finger can also have comparable effects. It is the function of these effector domains which are arguably the most important with respect to the use of designer zinc-finger proteins for therapeutic gene modulation. If a methylase domain is bound to the designer zinc-finger protein, when the zinc-finger protein binds to the target DNA sequence an increase in methylation state of DNA in that region will subsequently result. Transcription rates