Although responses to immune checkpoint blockade (ICB), e.g. antibodies against PD-1 (programmed cell death protein 1), PD-L1 (programmed death-ligand 1) and CTLA-4 (cytotoxic T-lymphocyte-associated protein 4), are seen across many solid tumours, the proportion of patients that benefit varies widely by cancer type and we currently lack biomarkers with which to reliably predict immunotherapy response3. The gene discussed is PDCD1; the disease is cancer.