Figure 3. The role of MYC in development and cancer.

During normal development, MYC levels start out high and decline in aging tissues, with a concomitant decline in both proliferation and apoptosis (left). During the development of cancer, MYC levels are increased and so is neoplastic growth, which compensates for the increase in apoptosis and results in tumorigenesis (right). Both proliferative and apoptotic thresholds are not absolute and vary in individual neoplasms, resulting in inter-tumor heterogeneity. Here tumors A, B, and C have already achieved their own maximum tolerated levels of MYC, and further increases, however small, would push them over the edge when combined with chemotherapy.