Regions of DNA that are unusually rich in dA.dT base pairs play a key role in the structure and function of the eukaryotic genome. AT-rich sequences are found within origins of replication in both plants and animals (Eckdahl et al. 1989; Eckdahl and Anderson 1990), within the scaffold associated regions (SARs) that anchor chromatin to the nuclear matrix (Gasser and Laemmli 1986) and upstream of yeast (Struhl 1985) and mammalian (Fashena et al. 1992) promoters, where they serve as transcriptional activators.