(PGD) Genetic diagnosis of an IVF embryo before embryo transfer. Made possible by removing one or two cells of the embryo (embryo microbiopsy). Typically performed on day 3 of embryo development in vitro, at the 8-cell stage, prior to compaction, or on day 5, as an expanding blastocyst.

Process by which the separate cells of the early embryo fuse together to form, for the first time, a tissue. Typically occurs at the stage called the morula. With compaction, it becomes difficult to detach a cell for preimplantation genetic diagnosis.

(CGH) A molecular DNA diagnostic technique whereby a set of chromosomes (a genome) is compared with a standard set using different colored dyes (typically red and green), so that any areas that do not exactly match will appear green or red instead of brown; a sensitive technique that can be applied to preimplantation genetic diagnosis as well as to chorionic villous sampling, or CVS, and to amniocentesis for prenatal diagnosis, yielding results much faster than is possible with a formal karyotype and more comprehensively than is possible with fluorescent in situ hybridisation.

The commonest of the alleles to result in cystic fibrosis (when two delta F508 alleles are present) or (can occur when one F508 allele is present in a man). A woman heterozygous for this gene (with one F508 allele) will be a carrier for cystic fibrosis (and for CBAVD) but will herself be otherwise normal. There are however many other faulty alleles that can also contribute to these clinical conditions. The most common alleles can be looked for at preimplantation genetic diagnosis (PGD).

A biopsy, or removal of one or two cells, from an early IVF embryo for preimplantation genetic diagnosis after in vitro fertilisation.

Known as "FISH". A form of DNA testing for genetic diagnosis in which a special region of a chromosome is stained with a dye that emits colored light when exposed to ultraviolet light. For example, a marker for chromosome 21 will normally show two spots of light, whereas three spots of light would indicate trisomy 21 (Down syndrome). Useful because the technique is accurate with just one cell, making diagnosis possible in an IVF embryo before transfer (see preimplantation genetic diagnosis), and speeding up prenatal diagnosis while awaiting a full karyotype. Being replaced in some labs by comparative genomic hybridisation.