Pulsed-field gel electrophoresis (PFGE) is one of the key technological advances of the past ten years that has made the mapping of genomes of whole organisms possible. In conventional electrophoreis, the mobility of DNA at almost any practical value of the field strength is essentially independent of mol wt above approx 30 kbp. Therefore, large (≥250 kbp) fragments of DNA required for mapping the genomes of entire organisms could not be separated prior to the introduction of these new electrophoresis techniques.