Increased PrPC expression stimulates the proliferation of regulatory T cells by up-regulating the altered growth factor-beta (TGF-b) and programmed death ligand-1 (PD-L1), which speeds up the progression of the tumor, according to a lung metastatic melanoma model in Prnp0/0 and Tga20 animals. The gene discussed is PRNP; the disease is neoplasm.