Some types of pulsed-field gel electrophoresis (PFGE), including CHEF (contour-clamped homogeneous electric field), give straight lanes and excellent separations of a wide range of DNA sizes on one gel (ref. 1 and Chapters 1 ,2 ,5 –7 ). Here conditions are described for the separation of DNA in the l0–,200-kbp size range by CHEF. CHEF resolution in this range exceeds that of many commonly used size standards. Production and use of a size standard giving a 10-kbp ladder in the l0–,200-kbp range (2 ) is described and examples of separations are shown.
