Like enhancers and promoters, insulators (or boundary elements) are a specialized class of regulatory DNA sequences. Insulators are defined by their ability to function in transgene assays to limit enhancer-promoter communication when placed between a promoter and an enhancer. They are also known to function as barrier elements that protect transgenes from chromosomal position effects. Two of the first insulator elements to be identified are the scs and scs' sequences which bracket two Hsp70 genes at the 87A locus of Drosophila. BEAF-32 was identified as a Boundary Element-Associated Factor of 32 kDa that binds to the scs’ insulator, and was subsequently shown to immunolocalize to hundreds of sites on polytene chromosomes, suggesting that BEAF-dependent insulators are a common feature in the Drosophila genome. This is substantiated by genome-wide mapping experiments, which have identified from 1800 to 6000 binding regions. The BEAF-32 gene encodes two isoforms, 32A and 32B, that differ only in their amino-terminal 80 amino acids. Both of these regions include a different atypical zinc-finger DNA binding domain. The remaining 200 amino acids are identical. BEAF-32 can form homo- and hetero- dimers, trimers and possibly also larger oligomers with varying ratios of 32A to 32B through interactions mediated by the sequences in the carboxy-end of the proteins. Yet transgene rescue experiments utilizing a lethal null BEAF-32 mutation found that 32B is essential, while 32A is not. In order to understand BEAF-32 better, we have used a combination of in vivo and in vitro experiments to map essential domains in the common portion of the proteins.