PDCD1 and skin squamous cell carcinoma: To better understand whether cancer cell plasticity and the presence of hybrid E/M and mesenchymal cancer cells could influence anti-PD-1/PD-L1 response, we analyzed the cancer cell features in a retrospective cohort of pre-treatment samples from unresectable locally advanced and metastatic cSCC patients, and in another cohort of stage III/IV HNSCC patients (Supplementary Tables 1 and 2).