The SMAD3 gene provides instructions for making a protein involved in transmitting chemical signals from the cell surface to the nucleus. This signaling pathway, called the transforming growth factor-beta (TGF-β) pathway, allows the environment outside the cell to affect cell function, including how the cell produces other proteins. The signaling process begins when a TGF-β protein attaches (binds) to a receptor on the cell surface, which activates a group of related SMAD proteins (including the SMAD3 protein). These SMAD proteins combine to form a protein complex, which then moves to the cell nucleus. In the nucleus, the SMAD protein complex binds to specific areas of DNA to control the activity of particular genes. Through the TGF-β signaling pathway, the SMAD3 protein also influences many aspects of cellular processes, including cell growth and division (proliferation), cell movement (migration), and controlled cell death (apoptosis).