Dr. Antonio Wolff and Dr. Kim Allison discuss the latest ASCO-CAP guideline update on HER2 testing in breast cancer. This guideline update affirms previous recommendations, and provides commentary based on data from the DESTINY-Breast04 trial. Dr. Wolff and Dr. Allison review the questions from the oncology and pathology community raised by these results, and provide commentary on patients with HER2 IHC 1+ and 2+ and ISH-negative metastatic breast cancer. Read the guideline update, "Human Epidermal Growth Factor Receptor 2 Testing in Breast Cancer: ASCO-CAP Guideline Update" at www.asco.org/breast-cancer-guidelines. TRANSCRIPT This guideline, clinical tools, and resources are available at www.asco.org/breast-cancer-guidelines. Read the full text of the guideline and review authors’ disclosures of potential conflicts of interest disclosures in the Journal of Clinical Oncology, https://ascopubs.org/doi/10.1200/JCO.22.02864   Brittany Harvey: Hello and welcome to the ASCO Guidelines Podcast, one of ASCO's podcasts delivering timely information to keep you up to date on the latest changes, challenges, and advances in oncology. You can find all the shows, including this one, at asco.org/podcasts.  My name is Brittany Harvey, and today I am interviewing Dr. Antonio Wolff from Johns Hopkins Sidney Kimmel Comprehensive Cancer Center and Dr. Kim Allison from Stanford University School of Medicine, co-chairs on ‘Human Epidermal Growth Factor Receptor 2 Testing in Breast Cancer: ASCO-CAP Guideline Update’. Thank you for being here, Dr. Wolff and Dr. Allison. Dr. Antonio Wolff: Thank you. Dr. Kim Allison: Thanks for having us.  Brittany Harvey: Then, before we discuss this guideline, I'd like to note that ASCO takes great care in developing its guidelines and ensuring that the ASCO conflict of interest policy is followed for each guideline. The disclosures of potential conflicts of interest for the full guideline panel, including our guests on this episode today, are available online with the publication of the guideline and the Journal of Clinical Oncology, linked in the show notes.  So now jumping into the content, to start us off, Dr. Wolff, what prompted the update expert panel to revisit the 2018 ASCO-CAP recommendations on HER2 testing and breast cancer, and what is the scope of this update? Dr. Antonio Wolff: Thank you, Brittany. We appreciate the opportunity of being with you today, and it's great to be here with my colleague, Dr. Kim Allison, as well.   What triggered this informatory update was the release from data in trial DESTINY-Breast04, which tested the antibody-drug conjugate trastuzumab deruxtecan in patients who in the past would have been considered to have HER2-negative disease. This ADC, trastuzumab deruxtecan, has a topoisomerase inhibitor payload that is linked to the antibody trastuzumab. And in the past, from all the previous data we had, trastuzumab alone, or in combination with chemotherapy, or as part of another antibody-drug conjugate T-DM1 was essentially active in patients with HER2-positive disease, which is traditionally defined as having overexpression of the HER2 protein, which almost by default is a result of gene amplification of the HER2 gene.  And what data from initial studies appear to suggest is that patients who would not be traditionally considered HER2-overexpressed or HER2-amplified were potentially benefiting or having evidence of clinical activity against this disease in the study of metastatic disease. And this was a randomized clinical trial for patients with metastatic breast cancer whose tumors were centrally determined to have IHC 1+ or IHC 2+ expression and would not have been called HER2-positive, would not have been called HER2-overexpressed. And for the tumors that were HER2 2+, they also had to have absence of gene amplification by an in-situ hybridization assay.  And what was very interesting is that there was a meaningful, clinically significant improvement in survival for tha