SMADs, or SMAD proteins, are a group of proteins that play a crucial role in the TGF-β signaling pathway. They are involved in transmitting signals from the cell surface to the nucleus, influencing various cellular processes such as growth, differentiation, and apoptosis. SMADs are categorized into three main types: receptor-regulated SMADs (R-SMADs), common SMADs (Co-SMADs), and inhibitory SMADs (I-SMADs). When a TGF-β ligand binds to its receptor, R-SMADs are activated and form complexes with Co-SMADs. These complexes then translocate to the nucleus, where they regulate the expression of target genes. I-SMADs can inhibit this process, providing a regulatory mechanism to control the signaling pathway and maintain cellular homeostasis.