PTGDR2 and neoplasm: Prostaglandin D2 (PGD2) synthase (PTGDS) and its receptor PTGDR2 are negatively correlated with stem genes (Sall4 and Lgr5) in gastric cancer, which restricts tumor self-renewal, growth, and metastasis by relying on the PTGDR2 pathway to inhibit STAT3 phosphorylation and nuclear expression (28).