The common mediator Smad, also known as Co-Smad or Smad4, is a crucial component of the [TGF-β signaling pathway]. It acts as a central mediator that transduces signals from the cell surface to the nucleus. Co-Smad is distinct from receptor-regulated Smads (R-Smads) and inhibitory Smads (I-Smads) because it facilitates the formation of a complex with R-Smads, which then translocates to the nucleus to regulate gene expression.