Condensation of long double-helical DNAs is a sharp phase transition, which takes place within a narrow interval of condensing agent concentrations.[ref] Since the double helices come very closely to each other in the condensed phase, this leads to the restructuring of water molecules, which gives rise to the so-called hydration forces.[ref] To understand attraction between negatively charged DNA molecules, one also must account for correlations between counterions in the solution.[ref] DNA condensation by proteins can exhibit hysteresis, which can be explained using a modified Ising model.