Aneuploidy is the presence of an abnormal number of chromosomes in a cell, for example when having 45 or 47 chromosomes when 46 is expected in a human cell. It does not include a difference of one or more complete sets of chromosomes, which is called euploidy. An extra or missing chromosome is a common cause of genetic disorders, including some human birth defects. Some cancer cells also have abnormal numbers of chromosomes. Aneuploidy originates during cell division when the chromosomes do not separate properly between the two cells.