In spite of the large amount of sequence conservation among the DNA binding segments of basic region leucine zipper (bZIP) proteins, these proteins can discriminate differently between target sequences that differ in half-site spacing. Here it is shown that the half-site spacing preferences of bZIP proteins are the result of (i) the differential intrinsic curvature in target binding sites that differ by insertion or deletion of a single base pair and (ii) the ability of some bZIP proteins to overcome this intrinsic curvature through a mechanism dependent on basic segment residues.