CC-4047 blocks development of osteoclasts at a very early time point of hematopoietic lineage commitment. Nonadherent cells from (A) HBM and (B) MMBM were cultured in methylcellulose culture containing 30% FCS and 100 pg/mL rh CSF-GM in the presence of 100 μM CC-4047, 100 μM thalidomide (Thal), or 0.1% DMSO. After 9 to 10 days, hematopoietic precursors were purified and used to set up an OCL formation assay as described in “Materials and methods.” During the 21-day culturing time, none of the drug was added to the culture. Each value is the mean plus or minus SD of mononucleated cells per well of at least 8 wells. Asterisks indicate a significant difference from control (P < .01). All experiments were performed independently twice.