SMAD1 (UniProt/Swiss-Prot entry Q15797) is a transcriptional modulator that mediates multiple signaling pathways, such as those of the bone morphogenetic proteins (BMPs). BMPs are involved in a range of biological activities including cell growth, apoptosis, morphogenesis, development and immune responses. SMAD1 can be phosphorylated by the BMP receptor kinase, leading to its activation.