Although PD-1/PD-L1 immune checkpoint blocking antibodies have demonstrated durable clinical responses in some patients, the majority of patients fail to respond to such a treatment. Histological staining of anti-PD-1-resistant tumors from a mouse model for lung cancer showed that the resistant tumor had significantly increased mitosis, more nuclear pleomorphism, and less immune cell infiltration than did the anti-PD-1 responsive tumor. For details, see article by Wang and colleagues on page 839