Cancer is a growth disorder of cells. It starts when an apparently normal cell begins to grow in an uncontrolled and invasive way. The result is a cluster of cells, called a tumor, that constantly expands in size. Cells that leave the tumour, spread throughout the body, forming new tumours at distant sites. This is called metastasis. Cancer is a gene disorder of somatic tissue, in which damaged genes fail to properly control cell proliferation. A series of several mutations to certain classes of genes is usually required before a normal cell will transform into a cancer cell. All cancers are characterised by unrestrained growth and division. Cell division never stops in a cancerous cells. Cancer cells are virtually immortal-until the body in which they reside dies.