CD274 and neoplasm: However, PD-L1 expression has been criticised as an imperfect predictive biomarker due to intratumoural and intertumoural heterogeneity [5], leading to the investigation of numerous other biomarkers including tumour mutational burden (TMB) and microsatellite instability (MSI) as surrogate biomarkers for tumour antigenicity and/or PD-L1 expression [6,7].