PDCD1 and neoplasm: After assessing the correlation between MS4A expression levels and immunoinhibitors, we found a strong correlation between MS4As and IDO1, PDCD1, or TGFB1. Indoleamine 2,3-dioxygenase 1 (IDO1) is known to cause immunosuppression through breakdown of tryptophan in the tumor microenvironment.