STAT3 and neoplasm: For example, the activity of CTLs is significantly dampened by immunosuppressive mediators produced by tumor cells such as indoleamine 2,3-dioxygenase 1 (IDO-1), programmed death-ligand 1 (PD-L1), cyclooxygenase type 2 (COX‐2), and signal transducer and activator of transcription 3 (STAT3) 32.