In vitro nucleosomal arrays assembled on the IgH enhancer. The rate and extent of the chromatin reconstitution reaction (detailed in Methods and Materials) on the Eμ+MARs template (Fig. 1C) was monitored by formation of nucleosomes. Aliquots were removed at regular 2–240 min intervals, digested with MNase for 5 min, fractionated on a 1.5% agarose gel, and then stained with ethidium bromide. Outside lanes, naked DNA; -ATP lanes, no ATP regeneration system.