Many DNA binding sites are symmetrical

The DNA base sequences of (a) the lac operator, to which the Lac repressor binds, and (b) the CAP-binding site, to which the CAP–cAMP complex binds. Sequences exhibiting twofold rotational symmetry are indicated by the colored boxes and by a dot at the center point of symmetry.

[(a) Data from W. Gilbert, A. Maxam, and A. Mirzabekov, in N. O. Kjeldgaard and O. Mall0e, eds., Control of Ribosome Synthesis. Academic Press, 1976]