Fact 1: Cancer is a disease that occurs when the cell cycle is no longer regulated.

Fact 2: Brain tumors are one of the fastest growing cancers today.

Given the two facts above, answer the question "Brain tumors occur when what is no longer regulated?" with the following options: 
- fossil fuels
 - headaches
 - voltage
 - heat or cold
 - cell cycles
 - critical organs
 - cerebellum
 - migraines


 

     cell cycles    