Agarose gel electrophoretic profiles of nested deletion time points. Purified pSCANS 14-5c DNA was digested with I-CeuI and PmeI and then incubated with ExoIII to initiate 3' to 5' digestion at the PmeI site. Aliquotes were then removed at the indicated times and treated with S1 nuclease to remove the single-stranded tails. M1 and M2 are restriction fragment size markers; lane 1 is the I-CeuI/PmeI cut DNA at 0 time; lanes 2-7 are samples taken at 30 sec. intervals and 8-13 are after 1 minute intervals, respectively; V is a mixture of super-coiled and linearized pSCANS vector without insert.