This is a phase IV, prospective, multicenter, non-interventional study (regulatory post-marketing surveillance) in approximately 500 patients who are to receive Kadcyla (trastuzumab emtansine). Patients administered Kadcyla infusion at physician's discretion will be registered for this surveillance in Korea. Patients will be asked to provide informed consent; data will be collected by electronic Case Report Forms for approximately 8 years.

Inclusion Criteria:

• Patients who are administered with Kadcyla at physician's discretion will be registered for this study. The use of Kadcyla in the patients must fall into the approved indication in Korea.

Exclusion Criteria:

• Patients not receiving treatment for breast cancer with Kadcyla according to standard of care and in line with the current summary of product characteristics (SPC)/local labeling.
• A pediatric patient (age ≤18 years)
• Hypersensitivity for Kadcyla or any ingredient in this product

Korean patients administered Kadcyla infusion at physician's discretion