While DNA testing has exonerated a lot of prisoners locked up for crimes they didn't commit, it isn't a fail-safe that will prevent the innocent from being executed. Capital Punishment Project Director John Holdridge wrote about why further measures must be taken to overhaul the justice system's prosecution of defendants :
...in the vast majority of criminal cases, there is no DNA evidence to test and possibly exonerate innocent prisoners. The best estimate is that DNA evidence exists in only 10 to 15 percent of all murder cases.
The all-to-common story of an reminds us that as long as prisoners continue to be found innocent, whether through DNA testing, or other means, there should never be an irreversible punishment such as death.