(a) Structure of the complex of λ repressor  with operator DNA. The protein was crystallized with a 19-bp duplex of which the central 17 bps are shown. The consensus half is to the left, (b) Relative free energy changes in the binding of λ repressor to O
1 on base substitutions. The figure shows the change in affinity that results from each of the three possible substitution at all 17 sites. The left part represents the consensus half-operator (solid box) and the right half the non-consensus half-operator (redrawn from ref. ).