Distributed amongst the 46 chromosomes are approximately 300,000 - 500,000 genes. The genes contribute to how we grow, what we look like, and how we behave. In normal cells, the chromosomes reproduce every time the cell divides. Occasionally, something goes wrong, and a number of genes are altered. When this happens, the cells escape from the normal controls of growth, and they multiply until they form a mass of cells, usually referred to as a tumor.
Some tumors are benign and are composed of cells which resemble the normal cells of that organ.
Although they may form a large mass, they do not spread. Metastasis is the word used to define the spreading of such a tumor which is called cancer. Cancers that develop in the various organs manifest themselves in different ways, which is why each cancer has a unique way of being diagnosed. For example, breast cancer is detected by changes seen on mammograms, and cervical cancer usually produces abnormalities detected by Pap smears. Once a cancer is diagnosed, it is given a stage indicating how advanced the tumor is.