Nuclear export factor for BMP-specific SMAD1/5/8 that plays a critical role in terminating BMP signaling and regulating mesenchymal stem cell differentiation by blocking osteoblast differentiation to promote myogenic differention. Directly recognizes dephosphorylated SMAD1/5/8 and mediates their nuclear export in a Ran-dependent manner. Interacts with SMAD1, SMAD5 and SMAD8.