When the reproductive rate of cells exceeds their death rate, the tissue becomes enlarged, forming a tumor. Although these cells are initially identical to the others in the tissue, they gradually take on characteristics of malignancy. The cancer cells reproduce rapidly and tend to be abnormally large or small. Malignant tumors grow very quickly and invade other tissues. Cancer types are named for the location of the tissue that gives rise to the tumor and the organs involved. Genetics, viruses, or even environmental exposure to substances like those in cigarette smoke may cause tumor formation. However, not all tumors are malignant; tumors that grow within a well-defined capsule are benign and unlikely to be life-threatening.