This SMAD1/5 arm of TGF-β signaling has also been shown to occur in a wide range of other cell types, including epithelial cells, fibroblasts and cancer cell lines, which do not express ACVRL1 (Liu et al., 1998; Daly et al., 2008; Liu et al., 2009; Wrighton et al., 2009). Here, SMAD1 is linked to cancer.