The majority of human cancers result from an accumulation of somatic mutations. Somatic mutations are not passed on to the next generation. An 80-year cancer-free lifespan is no small accomplishment. It requires as many as 10 million billion body cells to copy themselves correctly. It is easy to see how random errors can occur. These changes are acquired during a person's lifetime from exposures to carcinogens and other mutagens, or from random unrepaired errors that occur during routine cell growth and division. Occasionally, one of these somatic mutations alters the function of some critical genes, providing a growth advantage to the cell in which it has occurred. A clone then arises from that single cell.