To study the relevance of the GPR50/TβRI complex on cell proliferation and tumor development we choose the MDA-MB-231 breast cancer cell line in which we were able to recapitulate the spontaneous Smad2 and Smad3 phosphorylation in the presence of GPR50 (Supplementary Fig. 5a). Here, TGFBR1 is linked to breast carcinoma.