In this section, we examine in more detail how the deregulation of growth-promoting and growth-inhibiting signaling pathways contributes to tumorigenesis. We first discuss how mutations that result in the unregulated, constitutive activity of certain proteins or in their overproduction promote cell proliferation and transformation. Next we discuss how loss-of-function mutations in differentiation pathways contribute to tumorigenesis. We end this section with a description of how misregulation of genes that control programmed cell death, such as p53, drives tumorigenesis.