A drug used to treat HER2-positive breast cancer. It is used in certain patients who have already been treated with an anticancer drug called trastuzumab and a type of drug called a taxane. It is also being studied in the treatment of other types of cancer. Ado-trastuzumab emtansine contains a monoclonal antibody called trastuzumab that binds to a protein called HER2, which is found on some breast cancer cells. It also contains an anticancer drug called DM1, which may help kill cancer cells. Ado-trastuzumab emtansine is a type of antibody-drug conjugate. Also called Kadcyla and T-DM1.