ABSTRACT
The body of an animal operates as a society or ecosystem, whose individual members are cells that reproduce by cell division and organize themselves into collaborative assemblies called tissues. Cancer cells are defined by two heritable properties: they reproduce in defiance of the normal restraints on cell growth and division, and they invade and colonize territories normally reserved for other cells. Cancers are traditionally classified according to the tissue and cell type from which they arise. Carcinomas are cancers arising from epithelial cells, and they are by far the most common cancers in humans. One way of proving clonal origin is through molecular analysis of the chromosomes in tumor cells. If a single abnormal cell is to give rise to tumor, it must pass on its abnormality to its progeny: the aberration has to be heritable. For those cancers known to have a specific external cause, the disease does not usually become apparent until long after exposure to causal agent.
