3. Tumorigenesis is traditionally viewed as a disorder of excessive cellular proliferation. However, modern oncology recognizes that the accumulation of cell mass is equally dependent on the evasion of programmed cell death. Critically analyze the role of apoptosis (programmed cell death) in maintaining tissue homeostasis. Explain how a defect in the apoptotic pathway can lead to tumor formation even if the rate of cell division remains normal. In your response, provide one example of a molecular 'checkpoint' or protein (such as p53 or the Bcl-2 family) that regulates this balance.