A drug used to treat HER2-positive breast cancer that has spread to other parts of the body. It is used in patients who have already been treated with the anticancer drug called trastuzumab and a type of drug called a taxane. It may also be used in patients whose cancer has recurred (come back) after adjuvant therapy with these drugs. 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.