While only a few studies analyzed the prognostic and predictive potential of Treg count in MPM, others investigated the cytokines responsible for Treg recruitment and activation, such as transforming growth factor (TGF-β) [6,52,53,54] and cyclooxygenase-2 (COX-2)/prostaglandin E2 (PGE2) [39,40,41,42,55,56], which are released by cancer cells directly or indirectly by cancer associated fibroblasts [50]. The gene discussed is PTGS2; the disease is cancer.