Abstract
In this chapter, we will understand that the observation that RNA tumor viruses can transform normal cells into cancer cells led to the discovery of oncogenes. These genes have captured host genes encoding for key proteins in signal transduction pathways. Oncogenes can be activated by point mutations, translocations and amplifications resulting in the enhanced activity of signal transduction. This disconnects the control of cellular growth, metabolism and survival from exogenous signals, such as growth factors, cytokines, hormones and neighboring cells and leads to selective growth advantages of the cancer cells. Thus, the aberrant activation of oncogenes and their signaling networks allows transformed cells to acquire key hallmarks of cancer, such as evading growth suppressors, sustaining proliferative signaling and resisting cell death. The knowledge of tumor-specific genetic rearrangements is used in the diagnosis of cancer and in therapy monitoring but also holds prognostic information. Moreover, these molecular changes present therapeutic targets.