TGFB1 and cancer: Affinity pull-down followed by mass spectrometry revealed that Smad4, the central intracellular mediator of transforming growth factor-β/Smad (TGF-β/Smad) signaling, which is a key pathway in cancer invasion and metastasis, was one of the proteins associated with circPTEN1 with the highest abundance (Fig. 4A, Supplementary Fig. 8A and B).