PDCD1 and neoplasm: In humans, surrogate markers like programmed death 1 (PD-1, also known as CD279) and B and T lymphocyte attenuator (BTLA) have been used to enrich the anti-tumor response in blood because of associations with exhaustion in cancer (Gros et al., 2016; Gros et al., 2014; Huang et al., 2019; Huang et al., 2017; Kamphorst et al., 2017; Twitty et al., 2020; Yan et al., 2018).