Final answer:
p53 was proved to be a tumor suppressor gene because it normally halts the cell cycle for DNA repair or triggers apoptosis, but mutated p53 is found in over 50% of tumors and leads to uncontrolled cell growth.
Step-by-step explanation:
The protein p53 has been proved to be a tumor suppressor gene (TSG) given its crucial role in monitoring DNA integrity and responding to cellular stress. In healthy cells, p53 functions at the G1 checkpoint to halt the cell cycle, initiate DNA repair, or trigger apoptosis if the DNA damage is irreparable. However, when p53 is mutated, as found in over 50 percent of human tumors, it can no longer perform these functions, leading to unregulated cell proliferation and potential tumorigenesis. This malfunction in p53 is dramatically illustrated by a condition called Li-Fraumeni syndrome (LFS), where individuals with a mutated p53 allele have a significantly increased risk of cancer from an early age. Furthermore, laboratory studies with mutagenized p53 genes show characteristics typical of cancer cells, including uncontrolled growth and suppressed apoptosis, confirming the role of p53 as a TSG.