Signal transduction associated with growth factor receptors typically mediates the activation of cell cycle promoting gene products or the inactivation of cell cycle checkpoints. These signals are frequently transduced through G-Protein pathways, kinase receptor pathways, or nuclear receptor pathways (Fig. 4.1), are dependent on proto-oncogenic transcription factors, and lead to the expression of Cyclins and Cyclin-Dependent Kinases (CDKs), which are drivers of the cell cycle (Fig. 4.2). Physiologically, growth signals are transient and entirely depend on the engagement of a growth factor receptor by its cognate ligand. Upon termination of this interaction, the growth signal ceases. Gain-of-function mutations in cancer keep the growth signal active, regardless of the presence of the transient growth factor-receptor interaction. Such deregulated signaling cascades are appropriate drug targets in the treatment of cancer.