Final answer:
When a retrovirus inserts into the host genome next to a cellular oncogene, it can result in overexpression of the oncogene, uncontrolled cellular proliferation, and the development of cancer.
Step-by-step explanation:
When a retrovirus inserts into the host genome next to a cellular oncogene, it can have a significant impact on the host cell. The viral genome insertion can cause overexpression of the adjacent oncogene, leading to uncontrolled cellular proliferation and the development of cancer. Retroviruses have a higher chance of inserting near proto-oncogenes, but the overall chance is still low, resulting in long tumor latency compared to viruses that carry their own viral oncogene.