The principle of PGD by FISH is that target-specific DNA probes labelled with different fluorochromes or haptens can be used to detect the copy number of specific loci, and thereby to detect chromosome imbalance associated with meiotic segregation of chromosome rearrangements which includes the Robertsonian translocations, reciprocal translocations, inversions, and complex rearrangements. FISH can also be used to select female embryos in families with X-linked disease Polymerase chain reaction (PCR) has been used to diagnose monogenic disorders. PCR is used to amplify sufficient DNA from embryo cells . A blastomere is placed in a solution that lyses the cell and releases the DNA and the PCR reaction mix is then added to begin the PCR. Because of its high sensitivity, contamination of the study sample with extraneous DNA is a danger and has led to the adoption of rigorous laboratory procedures and standards, such as the use of intracytoplasmic sperm injection.