Transcriptional modulator activated by BMP (bone morphogenetic proteins) type 1 receptor kinase. SMAD1 is a receptor-regulated SMAD (R-SMAD). May form trimers with another Smad1 and the co-Smad Smad4. In the cytoplasm in the absence of ligand. Migration to the nucleus when complexed with SMAD4. Colocalizes with LEMD3 at the nucleus inner membrane. Phosphorylated on serine by BMP type 1 receptor kinase. Belongs to the dwarfin/SMAD family.