The genetic diagnosis technique which may yet save Molly's life

Molly Nash may owe her life to a technique known as preimplantation genetic diagnosis (PGD) which was used to screen embryos to select a brother with a compatible tissue type, lessening the risk of transplant rejection.

Molly Nash may owe her life to a technique known as preimplantation genetic diagnosis (PGD) which was used to screen embryos to select a brother with a compatible tissue type, lessening the risk of transplant rejection.

PGD can be performed only on embryos created by in vitro fertilisation, when a sperm and egg are fused in the test tube to create a fertilised egg, which is then allowed to develop for a further two or three days.

The technique involves taking two cells from the embryo when it consists of between six and 10 cells. Each cell is tested independently to confirm the accuracy of the procedure.

Countless animal studies suggest that removing up to two cells from an embryo at such an early stage of development has no side-effects, although doctors are warned to carry out long-term monitoring to show the same is true for humans.

Sometimes, an embryo biopsy can be done at a later stage, when the embryo is five days old and consists of scores of cells. However, although this makes testing easier as many more cells can be removed, embryos at this later stage have a lower chance of survival.

There are two main genetic tests undertaken in PGD. One scans for chromosomal abnormalities, such as Down's syndrome. This test can also be used to screen for sex-linked disorders, such as haemophilia, which affects only boys.

The other test uses the latest techniques in DNA analysis to investigate whether the embryo is affected by a single-gene disorder, such as cystic fibrosis.

The screening of Molly's younger brother was done thanks to improved molecular techniques used to analyse the genes involved in tissue rejection, the so-called HLA genes.

By comparing the HLA genes of Molly, her mother and her father, the scientists found the best match possible from the 15 embryos created.

The scientists at the Reproductive Genetics Institute in Chicago used a procedure that could analyse more than one HLA gene at the same time to screen the embryos for the ones that would be most compatible with Molly.

The result was Adam, who was born on 29 August. He donated blood cells from his umbilical cord to his older sister.