Pulse field gel electrophoresis

Historical Background Standard gel electrophoresis techniques for separation of DNA molecules provided huge advantages for molecular biology research. However, many limitations existed with the standard protocol in that it was unable to separate very large molecules of DNA effectively. DNA molecules larger than 15-20kD migrating through a gel will essentially move together in a size-independent manner. At Columbia University in 1984, Schwartz and Cantor developed a variation on the standard protocol by introducing an alternating voltage gradient to better the resolution of larger molecules. This technique became known as Pulsed Field Gel Electrophoresis (PFGE). The development of PFGE expanded the range of resolution for DNA fragments by as much as 2 orders of magnitude.
See more at Wikipedia.org...

(PFGE): A form of gel electrophoresis that allows extremely long DNA molecules to be separated from one another.