Consider, for example, preimplantation genetic diagnosis (PGD). This technique involves submitting embryos to genetic screening and then selecting the one—perhaps of the desired gender or free of a certain disease-causing gene—that is to be implanted into the uterus. Critics warn that PGD could lead to gender discrimination or that it might eventually be used to let couples choose other genetic traits for their children, including hair or eye color. PGD raises the ethical question, What happens to the embryos that are not selected?