For example in the process of pulmonary fibrosis, transforming growth factor β1 (TGF-β1) can bind to the let-7d promoter through Smad3, resulting in down-regulation of let-7d, which further up-regulates the high mobility group protein A2 (HMGA2) of its target gene, promotes EMT, and leads to fibrosis (Pandit et al., 2010). This evidence concerns the gene SMAD3 and pulmonary fibrosis.