PROM1 and osteosarcoma: CD133/prominin-1[21], a cancer stem cell marker, SSEA4 (stage-specific embryonic antigen 4)[22], a glycoprotein expressed early in embryonic development and in pluripotent stem cells, and SSEA1/CD15 (stage-specific embryonic antigen 1)[23] have been identified in osteosarcoma cells[13, 21–23].