All your cells contain proto-oncogenes, which can change into cancer-causing genes. Why do cells possess such potential time bombs? See Concept 18.5 (Page 386)

The protein products of proto-oncogenes regulate the processes of cell division and growth.

Explanation:

The proto-oncogenes are the genes that code for the proteins involved in the regulation of cell division, growth, and differentiation. Certain mutations in these proto-oncogenes transform them into the oncogenes.

For example, the "erbB" gene code for receptor of epidermal growth factor which in turn regulates cell division in epidermal cells. The mutated erbB oncogene code for truncated receptor for epidermal growth factor which in turn makes the cell to divide continuously.

Hence, the proto-oncogenes are required by cells to regulate the processes of cell cycle, growth, and differentiation.