Although therapy with immune checkpoint inhibitors (ICIs) for patients with non-small-cell lung carcinoma (NSCLC), which has recently become available, does offer a survival advantage compared with chemotherapy, the overall response rate is only around 20 %. Biomarkers are increasingly important in identifying patients who would benefit from ICI therapy. Expression of PD-L1 was the first predictive biomarker to be developed, but was unable to sufficiently predict the efficacy of ICI. Another biomarker, tumour mutation burden (TMB), is defined as the number of mutations per megabase of DNA analysed. Microsatellite instability also acts as a predictive marker for ICI therapy response. Many tumour entities demonstrate a high correlation between MSI and high TMB. Studies show a benefit of progression-free survival for patients with NSCLC and a TMB of at least 10 mutations per megabase.