Breast Cancers With Discordant Human Epidermal Growth Factor Receptor 2 Fluorescence In Situ Hybridization–Positive (Group 1) and Immunohistochemistry-Negative (0/1+) Results Following American Society of Clinical Oncology/College of American Pathologists Guideline

Dual IHC and FISH testing, especially from metastatic sites, may need to be considered to ensure all patients eligible for targeted therapies are identified. Significant differences in HER2 copy numbers between discordant and concordant cases raise the possibility of underlying biologic differences and require further investigation.