How two proteins mesh at their zipper regions was not initially obvious. If the regions were antiparallel, the leucines would be expected to interdigitate like the teeth in a true zippper. If the molecules were parallel, the opposing leucines would instead overlap. The parallel model, which places the DNA-binding regions in a suitable position for contacting dyad-symmetric (two-fold rotationally symmetric) motifs, is now known to be correct. The proteins combine to form what is called a coiled coil in the zipper region.