However, transient SMAD1/5 phosphorylation is not a defining characteristic of this arm of TGF-β signaling, as another human breast cancer line, BT-549, exhibited sustained SMAD1/5 phosphorylation that is still readily detectable 8 hr after TGF-β stimulation (Figure 1—figure supplement 1A). Here, SMAD1 is linked to breast cancer.