Bortezomib inhibits the proliferation of bone marrow stromal cells (BMSCs). MS-5 cells (A), BMSCs from 3 healthy individuals (B), and BMSCs from 5 myeloma patients (C) were incubated without or with bortezomib (5-500 nM) in 96-well plates in serum-free X-VIVO medium, and cell proliferation was measured by colorimetric assay. Data are the mean±SD of the relative proliferation indices from 3 independent experiments. (D) MS-5 cells were incubated with or without bortezomib for 24 hr prior to cell cycle analysis. ^a)P<0.05, as compared with the control (no bortezomib). (E) BMSCs from a normal individual, incubated for 24 hr, are shown.