Cancer cells are misregulated in their divisions because they continue dividing unchecked by how many cells are nearby or whether they are attached to the substrate. Also, they are often poorly self-checked at the various checkpoints for the successful completion of each stage of the cell cycle. These changes could be due to mutations in the various proteins, such as cyclins and Cdk's and receptors, which are responsible for the normal control of the cell cycle.
Work Step by Step
To get to this answer, go through the material on the cell cycle, paying special attention to checkpoints that determine how and when cells should replicate. These must not be functioning properly for the uncontrolled cell division seen in cancer to be possible.