While bone marrow‐derived mesenchymal stem cells (BM-MSCs) represent a highly utile cell type for cell transplantation therapy, the molecular network underlying BM-MSC proliferation remains poorly understood. Now, researchers led by Takeshi Teramura (Kindai University, Osaka, Japan) report TGFβ‐activated kinase (Tak1) as a critical regulator of BM-MSC proliferation.