PDCD1 and neoplasm: Tumor mutation burden (TMB) has the potential to serve as an important biomarker for predicting the response to immunotherapy, and its correlation with response rates to anti-PD-1 or anti-PD-L1 treatment has been confirmed in several types of tumors (Yarchoan et al. 2017; Lu et al. 2019; Chan et al. 2019).