A solution containing plasmid DNA, saturating amounts of ethidium bromide, and CsCl (44% w/v) is layered between two solutions of lesser (35% w/v CsCl) and greater density (59% w/v CsCl). During centrifugation to equilibrium, the closed circular plasmid DNA and linear DNAs form bands at different densities. - [Read Purification of Closed Circular DNA by Equilibrium Centrifugation in CsCl-Ethidium Bromide Gradient]