The hepatitis B virus EnhI enhancer element overlaps the promoter of the X gene. By performing methylation interference experiments, four protein factor binding sites clustered in a 120-bp region were found to control the EnhI enhancer and X promoter activities. Deletion mapping experiments indicated that the two upstream protein factor binding sites constituted a basal enhancer module. This module, likely bound by a liver-specific factor and a ubiquitous factor, could activate the herpes simplex virus thymidine kinase gene promoter by 5- or 10-fold, depending on the orientation, in Huh7 cells, a liver-derived cell line, but not in other cell types tested. The two downstream protein factor binding sites interact with the upstream basal enhancer module in an orientation- and distance-dependent manner to increase the enhancer activity by another 10-fold. In addition, at least one of the two downstream protein factor binding sites is also essential for the X promoter activity.