G1-S switch is defective. How viruses cause cancer a. Can carry an oncogene or inhibitor of a tumor suppressor b. Can integrate into DNA in middle of a normal gene and inactivate a tumor suppressor c. Can provide an enhancer or silencer that changes expression of critical host genes. You can inherit a predisposition to cancer, not the disease itself. If you are heterozygous for an inactive blocking gene, you can get cancer if second copy gets messed up.
Most cancer is sporadic, due to somatic mutation.
Most cancers not usually caused by virus.