Contour-clamped homogeneous electric field (CHEF) gelelectrophoresis is a particular formulation of pulsed-field gel electrophoresis (PFGE), which uses an array of electrodes positioned around the gel (on a contour) and clamped to specific voltages to produce a nearly homogeneous electric field inside the contour (1). The direction of the electric field is changed periodically, as with all pulsed-field techniques. In the case of CHEF, field reorientation is achieved electronically by changing the voltages (potentials) of the various electrodes in the array (see Fig. 1). Commercial CHEF devices currently employ a hexagonal electrode array, but other types of contours, such as circles or squares, if properly clamped, can also produce alternating homogeneous electric fields.