SMAD6 and idiopathic pulmonary fibrosis: For instance, miR-31 was reported to be considerably upregulated in the serum of IPF patients, compared with the level in healthy controls [63], and the levels of miR-31 expression were found to be positively correlated with the levels of SMAD2/AKT and SMAD6 expression in patients with IPF, while transforming growth factor-β (TGF-β) was found to induce miR-31 levels in A549 cells [64].