Trastuzumab emtansine is primarily used in the treatment of HER2-positive breast cancer, particularly for patients who have previously received trastuzumab and a taxane separately or in combination. It is specifically indicated for cases where the cancer has metastasized or recurred after prior therapy. The drug works by targeting HER2 receptors on cancer cells, delivering a cytotoxic agent directly to these cells, which helps to minimize damage to healthy tissues. This targeted approach makes it an effective option for managing advanced stages of HER2-positive breast cancer, improving progression-free survival and overall outcomes for patients.