Normally, proto-oncogenes stimulate the cell cycle. What are oncogenes and how do they affect the cell cycle

Oncogenes are mutated protooncogenes (see explanation below).

Explanation:

“Proto-oncogene” is the name given to various genes whose products stimulate cell proliferation. Examples may include growth factor receptors, and proteins involved in clearing cell cycle checkpoints. Mutations in a proto-oncogene may cause it to either become too rapidly transcribed and/or code for a product that functions in an unchecked manner. This may lead to unrestrained cell proliferation, a basic mechanism of cancer. When and if this mutation takes place, the proto-oncogene becomes known as an oncogene.