PRO-001 binds to FGFR3, inhibits downstream ERK1/2 phosphorylation, and induces apoptosis of primary t(4;14)⁺ MM cells. (A) Freshly isolated BMNCs were stained with PRO-001 (filled) or control antibody (open) and then stained with PE-conjugated anti–human secondary antibody. Myeloma cells were identified by CD138 labeling. (B) Primary myeloma cells were incubated in the absence (filled) or presence of FGF (gray line) or preincubated with 5 μg/mL PRO-001 (black line) for 2 hours and then stimulated with FGF. ERK1/2 phosphorylation was assessed by flow cytometric analysis. (C) Primary myeloma cells were cultured in the presence of control Fab (left panel) or 5 μg/mL PRO-001 (right panel). Cells were harvested after 7 days and stained with annexin V-FITC and analyzed by flow cytometry. Myelomas cells were identified as CD138⁺. The total percentage of CD138⁺ cells is shown in the top right quadrant. Shown is a representative experiment.