ER-ve breast cancer cell lines have been shown to be insensitive to the anti-proliferative effects of activin [11], however this effect does not appear to be due to low expression of the activin type II receptor, with evidence that MDA-MB-231 express activin type II receptors [11] and MDA-MB-436 have a functional activin-signaling pathway showing phosphorylation of Smad2 in response to exogenous activin following removal of follistatin from the medium [12]. Here, SMAD2 is linked to breast cancer.