Oncogenes are mutated forms of normal genes, known as proto-oncogenes, that play a role in cell growth and division. When these genes become altered, they can promote uncontrolled cell proliferation, leading to the development of tumors and cancer. These mutations can occur due to various factors, including environmental influences, radiation, or inherited genetic changes. Oncogenes are crucial in understanding how cancers, such as breast cancer and lung cancer, develop, and they are important targets for cancer therapies aimed at inhibiting their activity.