We investigated the effect of single-base mutation on the RNA-cleaving activity and ion-induced folding of 10-23 deoxyribozyme at the single-molecule level by 3-color ALEX FRET (alternating laser excitation fluorescence resonance energy transfer). We found that substitution or deletion of a single base in the active region of the enzyme leads to a different folding pathway and enzymatic activity for all three mutants studied, but the severity of the effect was dependent on the type of mutation and the mutation site. We suggest that mutation of even a single base may result in a considerably different ionic and hydrogen-bonding interactions. Structural changes of 10-23 deoxyribozyme as it successively binds with Mg(2+) and the substrate were also unambiguously identified by the current single-molecule-detection method.