In 1996, Science published an article that described a radical new technology for correcting genetic defects, one that appeared to be a million-fold more potent than previous approaches were. The publication caught the media9s attention, launched research projects around the world, and spawned a gold rush as researchers and entrepreneurs moved to stake their claim to the extraordinary promise of the technology. But the technique has turned out to be inconsistent or impossible to replicate in most labs that have tried it.