Smad proteins are a family of proteins that play a crucial role in the TGF-β signaling pathway, which is important for regulating various cellular processes such as growth, differentiation, and apoptosis. They act as intracellular mediators that transmit signals from the cell surface to the nucleus, influencing gene expression in response to external signals. When TGF-β binds to its receptor, it activates Smad proteins, leading to their phosphorylation. Once activated, these proteins form complexes that translocate to the nucleus, where they interact with other transcription factors to regulate target genes. This process is essential for maintaining normal cellular functions and development.