Read the following paragraph and answer the question. Give ONLY the answer, no other words or explanation.

For example:

Answer: <most likely answer, as short as possible>.

Paragraph: 
Mutations that lead to cancer usually occur in genes that control the cell cycle. These include tumor-suppressor genes and proto-oncogenes.


Question: Mutations that lead to cancer usually occur in genes that control the cell cycle. these include tumor-suppressor genes and what?
proto-oncogenes
proto-oncogenes.