With the increasing use of immunotherapy, methods of response testing need to be rigorous and comparable. However, as explained here by Fred Hirsch, MD, PhD, of University of Colorado, Denver, CO, the current response assays used differ both between drugs and in cut-off scores. Prof. Hirsch discusses the Blueprint PD-L1 IHC Assay Comparison Project, which aims to provide information on the analytical and clinical comparability of four PD-L1 IHC assays used in clinical trials. He covers the Phase I and II results and conclusions, and the next steps for the project. This video was recorded at the British Thoracic Oncology Group (BTOG) Annual Conference 2018, held in Dublin, Ireland.