Insulators, first identified in Drosophila, are DNA sequence elements that shield a promoter from nearby regulatory elements. We have previously reported that a DNA sequence at the 5′ end of the chicken β-globin locus can function as an insulator. It is capable of shielding a reporter gene from the activating effects of a nearby mouse β-globin locus control region element in the human erythroleukemic cell line K562. In this report, we show that most of the insulating activity lies in a 250-bp CpG island (core element), which contains the constitutive DNase I-hypersensitive site (5′HS4). DNA binding assays with the core sequence reveal a complex protein binding pattern. The insulating activity of the core element is multiplied when tandem copies are used. Although CpG islands are often associated with promoters of housekeeping genes, we find little evidence that the core element is a promoter. Furthermore, the insulator differs from a promoter in its ability to block the locus control region effect directionally.