For doctors who treat breast cancer this is a very exciting experimental new drug because it presents a whole new way of knocking out cancer cells.
The new drug appears to be superior, compared to the standard treatment, for women with advanced HER2-positive breast cancer who have not been helped by Herceptin.
The drug is given intravenously and is said to have low side effects.
Doctor Sandra Swain, an oncologist and president elect of the American Society of Clinical Oncology, said, "It's really a smart bomb. It's putting the chemo into exactly the cancer cell, and what it is, it binds to the cancer cell then dumps the chemotherapy in and does not affect the normal cells, so it's very exciting news."
T-DM1 is less toxic, and though it's not a cure, it does appear to delay the time in which cancer gets worse and seems to be improving survival.
This drug is not available yet, but experts say patients should talk to their oncologists about finding out if their eligible for clinical trials in which the drug is used.