1.74 X 10s M" 1 yield 8.6±0.2 mM base pairs. Cyclic voltammetric peak separations indicate that heterogeneous electron transfer is slower at DNA-modified electrodes than at unmodified glassy carbon electrodes. A prototype sequence-selective DNA sensor was constructed by immobilizing a 20-mer oligo (deoxythymidylic acid)(oligo (dT) 2o), following its enzymatic elongation with dG resi-dues, which yielded the species oligo (dT) 20 (dG) 98· Cyclic voltammograms of 0.12 mM Co (bpy) 33+ obtained before and afterhybridization withpoly-(dA) andoligo (dA) 20 show increasedcathodic peaks after hybridization. The single-stranded form is regenerated on the electrode surface by rinsing with hot, deionized water. These results demonstrate the use of electroactive hybridization indicators in a reusable sequence-selective bio-sensor for DNA.