The creation of "designer babies" to treat sick siblings is being challenged at the House of Lords.
The right donor could cure Zain
IVF technology allows doctors to select embryos whose tissue is perfect for use in a transplant operation.
However, campaigners want the Law Lords to ban the practice by overturning an Appeal Court decision from April 2003.
The row centres on the case of Zain Hashmi, six, from Leeds, who requires treatment for a debilitating blood disorder, betathalassaemia major.
To stay alive he currently has to have blood transfusions every month, and drugs fed by a drip for 12 hours a day.
The only way to cure him would be to give him a transplant of healthy blood cells to replace the defective cells produced in his bone marrow.
Following the Appeal Court ruling, Zain's parents, Raj and Shahana, went ahead with IVF, hoping to create a sibling whose tissue would match that of their sick son.
In theory, this would have allowed them to take stem cells from the new baby's umbilical cord and transplant them into Zain.
However, Mrs Hashmi has had a series of miscarriages.
The campaigning group Comment on Reproductive Ethics (Core) wants the Law Lords to examine the Human Fertilisation and Embryology Act 1990 - and to decide whether tissue typing of the sort used by the Hashmis is legal.
The High Court had imposed a ban on the treatment in December 2002 but this was overturned in the Court of Appeal.
Lord Brennan QC, representing Core founder Josephine Quintavalle, told the Law Lords they must decide whether the Human Fertilisation and Embryology Authority had exceeded its powers in granting a licence to permit tissue typing to test for compatibility.
He argued the Court of Appeal was wrong in concluding that tissue typing was a treatment service under the Embryology Act because the service was not done for the purpose of helping a woman to have a child.
Mrs Quintavalle says tissue matching is not legal
He said the embryo was chosen not for the benefit of that potential child but for the benefit of another person.
Shahana Hashmi told the BBC she hoped the Law Lords would back the Appeal Court ruling.
"We are hoping they will realise that all we are trying to do is to ensure that our children have a healthy and prolonged life."
But she backed tight regulation to ensure the technology was not abused by people who wanted to select embryos based on criteria such as hair colour, or intelligence.
Mrs Quintavalle told the BBC: "If the country wants these deliberately designed tissue matched babies, then it is for parliament to re-visit the Act."
The hearing was adjourned until Tuesday when a ruling will be reserved until a later date.