BMPR1A and plasma cell myeloma: To extend our investigation into the role of BMPs in myeloma, we next used a BMPR1a-Fc-solubilised ligand trap, which binds and neutralises BMP ligands that bind to the Alk3 (BMPR1a) receptor, including BMPs 2, 4, 5, 6, 7 and 825. Basal expression of Bmpr1a was higher than Bmpr1b in stromal progenitors and osteoprogenitors, although Bmpr1b expression was significantly increased in stromal progenitors from myeloma-bearing mice (Supplementary Fig. 8).