PCNA and neoplasm: Indeed, GPR55−/− mice exhibited an alteration of leucocyte population in the tumor microenvironment and, concomitantly, a reduced expression of pro-tumorigenic factors (e.g., cyclooxygenase 2 (COX-2), signal transducer and activator of transcription 3 (STAT3), and proliferating cell nuclear antigen (PCNA) [36].