ERBB2 and breast cancer: According to the 2018 ASCO/CAP guideline, reaffirmed by 2023 guideline update and supported by the 2023 ESMO Expert Consensus Statements on the definition, diagnosis, and management of HER2-low BC, it is recommended to perform HER2 testing on formalin-fixed paraffin-embedded (FFPE) tissue samples derived from primary invasive BC and/or recurrent/metastatic tumors [79].