Transcriptional modulator activated by BMP (bone morphogenetic proteins) type 1 receptor kinase. SMAD5 is a receptor-regulated SMAD (R-SMAD) (By similarity). May form trimers with the co-SMAD SMAD4. Cytoplasmic in the absence of ligand. Migrates to the nucleus when complexed with SMAD4 (By similarity). Belongs to the dwarfin/SMAD family.