The screening of early embryos for disease-causing genes to enable the selection of ‘healthy’ embryos. The technique is used in conjunction with in vitro fertilization, which typically yields a number of embryos, and currently is approved in the UK for testing over 400 conditions. A single cell is removed from an eight-stage embryo and subjected to genetic testing; for example, it may be tested for a specific disease allele using either a gene probe and fluorescence in situ hybridization or the polymerase chain reaction. If the results are satisfactory, the embryo is implanted in the mother’s uterus, and development proceeds. Removal of a single cell at this stage does not affect the embryo’s subsequent development. PGD can help especially when couples who are being treated for fertility problems also have a history of genetic disease. However, use of PGD can be extended in nontherapeutic ways, such as choosing a baby’s sex or selecting particular desired traits to produce so-called ‘designer babies’. These highly controversial applications of PGD are prohibited in certain countries, including the UK. Preimplantation tissue typing is a related technique used to select embryos that are an exact tissue match for a living close relative, e.g. a sibling, who is suffering from a life-limiting blood disorder such as beta thalassaemia or Fanconi anaemia. The aim is for the healthy ‘saviour sibling’ to provide, in due course, stem cells that will help to treat the sick relative.