Phosphodiesterase-5 (PDE-5) inhibitors, such as sildenafil or tadalafil, have been reported to reduce the expression of arginase-1 and iNOS and thus inhibit the immune escape mechanism mediated by MDSCs while restoring T cell proliferation and inducing tumor cell apoptosis [142,143,144]. The gene discussed is ARG1; the disease is neoplasm.