Long-range comparative sequence analysis provides a powerful strategy for identifying conserved regulatory elements. The stem cell leukemia (SCL) gene encodes a bHLH transcription factor with a pivotal role in hemopoiesis and vasculogenesis, and it displays a highly conserved expression pattern. We present here a detailed sequence comparison of 193 kb of the human SCL locus to 234 kb of the mouse SCL locus. Four new genes have been identified together with an ancient mitochondrial insertion in the human locus. The SCL gene is flanked upstream by the SIL gene and downstream by the MAP17 gene in both species, but the gene order is not collinear downstream from MAP17. To facilitate rapid identification of candidate regulatory elements, we have developed a new sequence analysis tool (SynPlot) that automates the graphical display of large-scale sequence alignments. Unlike existing programs, SynPlot can display the locus features of more than one sequence, thereby indicating the position of homology peaks relative to the structure of all sequences in the alignment. In addition, high-resolution analysis of the chromatin structure of the mouse SCL gene permitted the accurate positioning of localized zones accessible to restriction endonucleases. Zones known to be associated with functional regulatory regions were found to correspond precisely with peaks of human/mouse homology, thus demonstrating that long-range human/mouse sequence comparisons allow accurate prediction of the extent of accessible DNA associated with active regulatory regions.