Response rates to immune-checkpoint inhibitors vary dramatically between patients with different tumour types, although the underlying reasons remain largely unknown. Now, in an analysis of TCGA data, including data from >10,000 samples and 34 different tumour types, researchers explored the relationship between the level of PD-1 mRNA and sensitivity to ICIs. A positive correlation (r = 0.91) was observed between the percentage of tumours with a ‘high’ PD-1 mRNA level (defined as the proportion of samples above the 80th percentile for PD-1 mRNA level) and overall response rates to anti-PD-1 antibodies reported in the literature. Furthermore, high PD1 mRNA levels were strongly correlated with expression of CD8+ T cell-related genes. These observations were confirmed in an independent set of 773 tumour biopsy samples. These data demonstrate the potential of PD-1 mRNA as a biomarker of response to anti-PD-1 antibodies.