Fig. 5. Ribbon diagram of two symmetry-related copies of the human NTC (PDB code 2F8X), revealing the near-linear orientation of the two DNA elements, which mimics the inverted repeat of the SPS element. Superposition of the symmetry-related pseudo-dimer (DNA in yellow, with CSL-binding sites in orange) on ideal B-form DNA corresponding to 42 base pairs of the hes1 promoter (gray, with CSL-binding sites in black) reveals that the orientation and spacing between the two CSL sites in the crystal approximates, but does not match, the expected spacing and orientation in a natural SPS.