CLOCK and osteosarcoma: To determine if these NE genes passively receive cues from the core clock apparatus or if their protein products also actively play a role in maintaining circadian rhythms, we altered their protein levels in human osteosarcoma U2OS cells that express a luciferase reporter gene under the control of mouse Bmal1 promoter (Bmal1-Luc) and examined circadian period in cell culture (Vollmers et al., 2008).