CS1 knockdown blocks in vivo tumor formation. (A) Mice (n = 8 per group) received pools of CS1null OPM2 cells or control OPM2 cells. Tumor development (top panel) and human λ light chain (ng/mL) in blood (bottom panel) were compared (P < .01). ○ represents CS1null OPM2 cells; ●, cntOPM2 cells. (B) Survival was evaluated after cell inoculation until death (mice were killed when tumors reached 2 cm³ in volume). P < .01. (C) Flow cytometric analysis of injected cells and tumors in mice injected with CS1null OPM2 cells. Solid histogram represents CS1 expression; open histogram, isotype control.