Just-released research about a new class of drugs called “PARP inhibitors” is the most exciting development in cancer research in a decade or more. In just a few years it could save thousands of lives. In the longer term, the drugs could represent a transformational approach to understanding and treating several forms of the disease. All this enthusiasm is based on a small report published today in the New England Journal of Medicine. It focuses on one clinical trial in its earliest stage in 60 patients with breast, ovarian and prostate cancer. Some — but not all — of the...