As a means to obtain information about the interaction between DNA and
protein at the CRE site, a DNaseI footprinting experiment was performed. The size of the bands that resulted depended on the extent that the CRE site is bound by protein in the examined extract - the more of the site that is bound to protein, the larger size fragment that remains protected after DNaseI digestion and the shorter distance the fragment migrates on the gel. When Kandel and colleagues performed the DNaseI footprinting assay on parallel extracts from HeLa and <em>Aplysia</em> respectively, they found two retarded bands and three retarded bands; the protection from cleavage of the bands was due to binding of protein at or around the CRE site. The take-home message from this figure is that in cellular extracts purified from <em>Aplysia</em>, the model organism of choice, the CRE DNA must be bound to some protein component.