Selecting the gender of the embryos is a legal procedure in USA and is very easy to carry out by means of an embryo biopsy, followed by a type of complete chromosome screening as "PGD-AS" (Preimplantation Genetic Diagnosis of Aneuploidy Screen). To do this, several cells are removed from the outer layer of the embryo. The next step is to vitrify (freeze) the embryos while the extracted cells are analyzed in the laboratory. In this step, all the chromosomes of the embryos(s) are examined, which in turn will allow the sex of the embryos to be determined so that patients will finally have a chromosomally normal embryo of the desired gender.