Flexi Says: If the cell cycle occurred without regulation, cells might go from one phase to the next before they were ready. The cell cycle is controlled mainly by regulatory proteins. These proteins control the cycle by signaling the cell to either start or delay the next phase of the cycle at key checkpoints. Cancer is a disease that occurs when the cell cycle is no longer regulated. This may occur due to mutations in genes that code for regulatory proteins and in tumor suppressing genes. Cancer cells grow rapidly and may form a mass of abnormal cells called a tumor.