Though the exact cause is unknown, there are various theories on what could lead to the condition. The viral theory says that a cell becomes cancerous when infected by a virus. The immunological theory says that an abnormal cell survives to become a cancer because the body's defenses somehow break down. The chemical theory rests on the evidence from animal tests that certain chemicals cause cancer by altering the genetic make-up of a cell. These are known as carcinogens; tobacco smoke is one. Relation is also known to cause cancer. Cancer has many causes, some of which are still unknown.
However, research on cancer continues. Using the molecular data from the Human Genome Project, scientists at the Cancer Research Centre at Massachusetts Institute of Technology have identified the gene-rhoC-that causes cancer cells to break away from the original tumor and form new tumors in other parts of the body. This gene triggers the sequence of events that lead to a secondary tumor.