SPP1 and neoplasm: By contrast, osteopontin was expressed in ~ 39% of cells in the bones of tumor-bearing mice injected with MDA-MB-231GFP/Luc2 cells alone, when compared to the bones of tumor-bearing mice injected with either MDA-MB-231GFP/Luc2 cells plus EO-231 cells or MDA-MB-231GFP/Luc2 cells plus MC3T3-E1 cells (Fig. 3).