GDF2 and plasma cell myeloma: With these considerations we might be able to understand why Olsen et al. required only low concentrations of Activin A to inhibit BMP9 and BMP6-mediated inhibition of cell proliferation in INA6 and IH-1 myeloma cell lines, but could not detect any effect of similar levels of Activin A on BMP2/4 in IH-1 cells [39].