Aneuploidy is an abnormality in the number of chromosomes. Genome mutations involve alterations in the number of chromosomes. Due to irregularities during cell division, some chromosomes may not replicate properly or pairs of chromosomes may not segregate when the distribution into daughter cells takes place. The number of chromosomes thus increases or is reduced. PGD is used for the screening of trisomies 13, 18 and 21.