The balance between TGF-β1 and BMP signaling also plays an important role in pulmonary fibrosis, TGF-β1 and BMP signal through a heteromeric cell surface serine/threonine kinase complex, resulting in the receptor-mediated phosphorylation and activation of Smad2/3, or Smad1/5/8 (BMPs) transcription factors and alterations in gene transcription [37]. Here, SMAD1 is linked to pulmonary fibrosis.