Final answer:
Mutations are the largest contributor to the formation of cancer, transforming proto-oncogenes into oncogenes and deactivating tumor suppressor genes, leading to uncontrolled cell growth.
Step-by-step explanation:
The largest contributor to the formation of cancer is mutations. While proto-oncogenes and tumor suppressor genes play critical roles in the regulation of cell growth and division, it is the mutations in these genes that lead to cancer. These genetic alterations may convert proto-oncogenes into oncogenes that promote unregulated cell division, or they may inactivate tumor suppressor genes that would normally inhibit the growth of abnormal cells. Heredity can also play a role, as certain genes like BRCA1 and BRCA2, when inherited in a mutated form, can significantly increase the risk of developing specific types of cancer such as breast and ovarian cancer.