The human beta-globin locus control region (LCR) is essential for high-level expression of human epsilon-, gamma-, and beta-globin genes. Developmentally stable DNase I hypersensitive sites (designated HS) mark sequences within this region that are important for LCR activity. A 1.9-kilobase (kb) fragment containing the 5' HS 2 site enhances human beta-globin gene expression 100-fold in transgenic mice and also confers position-independent expression. To further define important sequences within this region, deletion mutations of the 1.9-kb fragment were introduced upstream of the human beta-globin gene, and the constructs were tested for activity in transgenic mice. Although enhancer activity was gradually lost with deletions of both 5' and 3' sequences, a 373-base-pair (bp) fragment retained the ability to confer relative position-independent expression. Three prominent DNase I footprints were observed in this region with extracts from the human erythroleukemia cell line K-562, one of which contained duplicated binding sites for transcription factor AP-1 (activator protein 1). When the 1.9-kb fragment containing an 18-bp deletion of the AP-1 binding sites was tested in transgenic mice, enhancer activity decreased 20-fold but position-independent expression was retained.