Each pair of chromosomes is numbered and can be identified by special microscopic techniques. The genes are located on the chromosomes and are determined by a specific sequence of nucleotides on a segment of the chain. The genes determine all of each individual's characteristics. They also control cell division and the repair of chromosomal defects. A defect on chromosome 13 is thought to be responsible for some cancers that run in families.
Cancer is an abnormal proliferation of cells that divide uncontrollably.
Normal cells divide only enough to replace those that are lost or to repair injuries, then stop dividing. Cancer cells have lost this restraint and keep dividing. The mass of cancer cells will eventually become large enough to cause problems and to become detectable. This will produce a lump, mass, or tumor that can be felt. The word oncology means the study of lumps.