The first step in the process is initiation, in which a change in the cell's genetic material primes the cell to become cancerous. However, not all cells are equally susceptible to carcinogens. A genetic flaw in the cell or another agent, called a promoter, may make it more susceptible. Even chronic physical irritation may make cells more susceptible to becoming cancerous. In the next step, promotion, a cell that has been initiated becomes cancerous.
Promotion has no effect on noninitiated cells.
Thus, several factors, often the combination of a susceptible cell and a carcinogen, are needed to cause cancer. In the process by which a normal cell becomes a cancerous cell, ultimately its DNA undergoes change. Changes in a cell's genetic material are often hard to detect, but sometimes a change in the size or shape of one specific chromosome indicates a certain type of cancer. For example, an abnormal chromosome called the Philadelphia chromosome is found in about 80 percent of the people with chronic myelocytic leukemia. Genetic changes have also been identified in brain tumors and cancers of the colon, breast, lung, and bone.