HER2-Predict is a multi-center, observational study using biological samples from patients treated with DS-8201a in the metastatic setting. Patients will provide a baseline FFPE tumor sample and additionally, blood sample for ctDNA determination will be collected.

A. Inclusion Criteria

1. Women and men ≥18 years old that has been included in a trial using Trastuzumab Deruxtecan (T-DXd; DS-8201a).

2. The participant (or legally acceptable representative if applicable) provides written informed consent for the study. It is possible to include patients that were exitus during or after treatment by filling out the appendix 1 form and filing it into the patient's clinical chart.

3. Patients who are starting, are receiving or have received treatment with Trastuzumab Deruxtecan as a treatment for metastatic/advanced cancer.

4. Submission of a formalin-fixed, paraffin-embedded tumor (FFPE) tissue block from the most recently collected tumor tissue, with an associated pathology report, for central molecular analysis and for other protocol-mandated secondary and exploratory assessments. If a newer specimen is either insufficient or unavailable, the patient may still be eligible if the patient can provide a tissue block (preferred) or is willing to consent to and undergo an additional pretreatment core or excisional biopsy (if it is assessable and the biopsy can be safely obtained). The tumor tissue should be of good quality and quantity based on total and viable tumor content.

   1. Acceptable samples include core needle biopsies of the deep metastatic lesion or excisional, incisional, punch, or forceps biopsies for cutaneous, subcutaneous, or mucosal lesions or biopsies from bone metastases.
   2. Fine needle aspiration, brushing, cell pellet from pleural effusion and lavage samples are not acceptable.

5. Patients included before starting experimental treatment must be able and willing to provide blood sample(s).

   B. Exclusion Criteria

6. Not having participated or not willing to participate in a trial using Trastuzumab Deruxtecan.

7. Inability to comply with study and follow-up procedures