Moreover, PSG9 enhanced the stability of Smad2, Smad3, and Smad4 proteins by blocking their proteasomal degradation, and regulated the expression of TGF‐β1 target genes involved in EMT and breast cancer progression, thus further amplifying the canonical TGF‐β/Smad signaling in breast cancer cells. The gene discussed is SMAD4; the disease is breast cancer.