Substitution of the conserved guanine outside of the canonical ERE disrupts ER binding. (a) Interactions between ER and wild-type and mutant EREs were measured by SPR. The canonical ERE is underlined, and the conserved guanine is indicated by an arrow. Base substitutions are indicated in bold. (b) Binding of ER to ERE is indicated as a percentage of binding relative to the wild-type sequence. ER, estrogen receptor; ERE, estrogen response element; SPR, surface plasmon resonance.