Experimental identification of functional genomic sequences depends heavily on cell culture and other techniques of in vitro cell biology. Traditionally, identification of gene regulatory regions has been limited by the labor-intensiveness of the requisite strategies. Generally only regions close to the transcriptional start sites have been experimentally tractable for detailed examination, despite evidence that important regulatory regions exist more than 10 kb upstream or downstream from the coding region or in introns of genes (Rowntree et al. 2001) .