SERPINE1 and metastatic melanoma: We first confirmed that primary and metastatic melanoma specimens express similar TGFB1 transcript levels (S4A Fig), and found a direct correlation with the expression of SERPINE1 (PAI-1), a downstream target of TGF-β signaling (which we initially used as the reporter gene in our TGF-β assay in vitro (Fig 1)) (Fig 2A).