There is a strong connection between the cell cycle and cancer. In cancerous cells the cell cycle no longer operates normally, and the resulting abnormal cells grow uncontrollably. These rapidly growing cells become tumors and cause damage to various organs and organ systems. Cancerous cells divide in the same manner as other cells, but because of an error in their genetic coding, they divide without the numerous safety measures that prevent normal cells from dividing too frequently.

All cells in the body divide to reproduce, and in healthy cells, a number of factors control whether or not a cell divides. Healthy cells are able to determine that there is enough space and nutrition to support two cells where there has only been one. They can also determine whether the deoxyribonucleic acid (DNA) has replicated properly. Also, normal cells will only divide when there are certain hormones present.

Cancerous cells are able to divide without the presence of hormones that normally regulate healthy cell division. These cells also do not respond to the proximity of other cells, which means that they are able to continue replicating even when there is no space for them to do so. Faulty DNA is ignored in cancer cells so that the cells continue the division process even when the daughter cells will be damaged. Tumors form as the unregulated cell cycle continues and produces rapidly growing abnormal tissue.

The connection between the cell cycle and cancer is that the cycle in cancerous cells has been disrupted. The abnormal cells no longer receive the instructions given to normal cells, including the instructions that tell them to divide and, importantly, the information that instructs them to die. The DNA of the damaged cells often lacks the gene responsible for producing a protein that inhibits cell division. Cancerous cells may also produce substances that are actively harmful to normal cells and that allow the cancerous cells to spread beyond the boundaries in which the normal cells were contained.

In order for an uncontrolled cell cycle and cancer to develop, the DNA in a cell must have mutated. Carcinogens can be responsible for creating these mutations. They can, however, also occur spontaneously or be inherited like other characteristics.