This gene encodes a protein in the SMAD family, which transduces signals and regulates transcription in various pathways. It mediates TGF-beta signaling, affecting cell proliferation, apoptosis, and differentiation. The protein interacts with the SMAD anchor for receptor activation (SARA) and is recruited to TGF-beta receptors. Phosphorylation by TGF-beta receptors causes dissociation from SARA and association with SMAD4, allowing translocation to the nucleus. There, it binds to promoters and forms a transcription repressor complex. The protein can also be phosphorylated by activin type 1 receptor kinase to mediate activin signaling. Alternative splicing results in multiple transcript variants.