The treatment builds on Herceptin, the first gene-targeted therapy for breast cancer. It is used for about 20 percent of patients whose tumors overproduce a certain protein.

Researchers combined Herceptin with a chemotherapy so toxic that it can’t be given by itself, plus a chemical to keep the two linked until they reach a cancer cell where the poison can be released to kill it.

This double weapon, called T-DM1, is the “smart bomb,” although it’s actually not all that smart — Herceptin isn’t a homing device, just a substance that binds to breast cancer cells once it encounters them…

The median time until cancer got worse was nearly 10 months in the women given T-DM1 versus just over 6 months for the others. That is about the same magnitude of benefit initially seen with Herceptin, which later proved to improve overall survival, too, Blackwell said.