An early view of eukaryotic chromosomes was that of static structures, which stored DNA not in use within a given cell type. It was thought that packaging of DNA into higher levels of chromatin structure would suffice to repress gene expression and that the challenge to the cell would be to rescue specific sequences from these structures. The exten- sive packaging of inactive DNA was considered the primary difference between eukaryotic and prokaryotic genomes and except for that point both would be similarly regulated by cis-acting sequences and trans- acting factors. Our view of eukaryotic...