In 2019, the US Food and Drug administration (FDA) approved the anti-programmed cell death (PD-1) monoclonal antibody, pembrolizumab, for the treatment of patients with relapsed or metastatic head and neck squamous cell cancers (R/M HNSCC). The PDL1-IHC 22C3 pharmDx kit was included as a companion diagnostic test to select patients with HNSCC with a combined positive score of ≥1 for treatment with pembrolizumab. PD-L1 expression on tumour and tumour infiltrating immune cells generally correlates with improved efficacy with immune checkpoint inhibitors (ICB) in multiple clinical trials.