Final answer:
Certain forms of cancer occur because of mutations in DNA sequences that are located in tumor-suppressor genes and proto-oncogenes.
Step-by-step explanation:
Certain forms of cancer occur because of mutations in DNA sequences that are located in tumor-suppressor genes and proto-oncogenes. Tumor-suppressor genes normally prevent cells with damaged DNA from dividing, but mutations in these genes prevent them from functioning properly, allowing cells with damaged DNA to divide. Proto-oncogenes normally help control cell division, but mutations can transform them into oncogenes that promote the division of cells with damaged DNA.