GREM1 and breast carcinoma: Irrespective of the precise mechanisms, our results demonstrate potent pro-tumorigenic effects of Grem1 on cancer cells and CAFs in vitro in mono- and in co-culture, as well as a key in vivo role for Grem1 in stimulating extravasation and for Grem1-producing CAFs in mediating the intravasation of breast cancer cells.