formation. About 20-30% of breast cancers overexpress HER2. Those patients may be candidates for the drug trastuzumab , both in the postsurgical setting (so-called " adjuvant " therapy), and in the metastatic setting. [ 49 ] HER2 status can be assessed by immunohistochemistry, fluorescent/chromogenic/silver in situ hybridization (FISH/CISH/SISH), or virtual karyotyping