Cruciform structures. A) A schematic representation of a cruciform is shown. An unpaired loop of 3-4 bp typically exists at the tip of the cruciform arms. B) Atomic force microscopy (AFM) image of a 106 bp inverted repeat extruded into a cruciform with 53 bp arms in the extended conformation in supercoiled plasmid DNA. C) AFM image of a 106 bp inverted repeat extruded into a cruciform with 53 bp arms in the X-type conformation.