First, technical reasons: different protocols for immunohistochemical staining use different antibodies with varying binding affinities, different scoring systems and criteria for positivity.13,14 Second, clinical reasons: scoring will depend on biopsy quality and inherently be subject to interobserver variability.15,16 Thirdly, biological reasons: PD-L1 expression varies within a given tumour.17,18 Expression of PD-L1 is usually assessed from single tumour biopsies, which will be affected by intratumor heterogeneity. The gene discussed is CD274; the disease is neoplasm.