What are cell cycle regulators? a. Stimulatory proteins are encoded by Examples include: I b. Inhibitory proteins are encoded by Examples include: 13. Cancer is the result of an improperly regulated cell cycle. Describe two reasons why cells can form tumors. 14. In some types of colon cancer, stem cells have a mutation in the APC gene. What happens if the APC gene is mutated? 15. Normally, proto-oncogenes stimulate the cell cycle. What are oncogenes and how do they affect the cell cycle? www.Biointeractie om a. To cause cancer, proto-oncogenes require therefore are considered function. allele(s) to be mutated and The mutation results in a of 16. Normally, tumor suppressor genes inhibit the cell cycle. How do mutated tumor suppressor genes affect the cell cycle? a. To cause cancer, tumor suppressor genes require allele(s) to be mutated and therefore are considered The mutation results in a of function.