The process by which embryos created by IVF are selected for implantation. The practice of preimplantation genetic diagnosis ( PGD) means that it is now possible to select an embryo that does not have, or is not likely to have, a particular disability, or to select an embryo with certain characteristics. PGD requires a licence from the Human Fertilization and Embryology Authority (HFEA), which will only allow the procedure where there is a significant risk of a serious genetic condition being present in the embryo. Licences have been granted to allow the selection of an embryo that is a genetic match to an existing child suffering from a genetic illness. Tissue from a child conceived in this way – popularly known as a saviour sibling – can be used to treat the older brother or sister. Sex selection, i.e. the selection of an embryo for implantation on the basis of gender, is only allowed by the HFEA to avoid a risk of a genetic illness, not for social reasons.