We observe reversible, bias-induced switching of conductance via a blue copper protein azurin mutant, N42C Az, with a nearly 10-fold increase at vertical bar V vertical bar > 0.8 V than at lower bias. No such switching is found for wild-type azurin, WT Az, up to vertical bar 1.2 V vertical bar, beyond which irreversible changes occur. The N42C Az mutant will, when positioned between electrodes in a solid-state Au-protein-Au junction, have an orientation opposite that of WT Az with respect to the electrodes. Current(s) via both proteins are temperature-independent, consistent with quantum mechanical tunneling as dominant transport mechanism. No noticeable difference is resolved between the two proteins in conductance and inelastic electron tunneling spectra at