Normally when a cell divides each daughter cell receives an equal number of chromosomes. When this does not happen, cells have incorrect numbesr of chromosomes. This is termed aneuploidy. For example, three number 21 chromosomes or trisomy 21 (characteristic of Down syndrome) is a form of aneuploidy. One of the theories of cancer is that it is set in motion by aneuploidy later in life when there is an unequal division of chromosomes during cell division. The champion of this theory is the famous Peter Duesberg.