SMAD1 and breast cancer: Notably, sEV TGF‐β+ did not significantly affect the closely related bone morphogenetic protein (BMP)‐SMAD1/5 signalling in breast cancer cells (Figure S1g), nor did it impact the interleukin 6 (IL‐6)/signal transducer and activator of transcription 3 (STAT3) signalling (Figure S1h).