A split luciferase-based sensor system was developed to noninvasively monitor and image phosphorylation-mediated c-Myc activation, in which the complementation of the split FL is induced by phosphorylation-mediated interaction between GSK3β and c-Myc. The complemented luciferase activity resulting from this interaction is specific to c-Myc phosphorylation and correlated with the steady-state and temporal regulation of c-Myc phosphorylation in cell culture. The sensor system also allows monitoring of c-Myc—targeted drug efficacy in intact cells and living animals. This new imaging sensor can provide insight into the role of functional c-Myc in cancer biology and is useful for the discovery and development of specific anti-c-Myc drugs.