Dr Andrew Wakefield, the doctor who first suggested a link between the MMR vaccine and autism, is facing a professional misconduct charge at the General Medical Council.
It marks the latest saga is the long-running controversy over the jab.
What is MMR?
MMR is a combined vaccine against measles, mumps and rubella, three common infectious diseases of childhood.
It was introduced in the UK in 1988 to replace single vaccines for each disease.
It is used in countries throughout the world, with millions of doses delivered each year.
Why were people worried about it?
In 1998, a study published in the respected journal The Lancet raised the possibility that the jab may be linked to autism and bowel disease.
The paper and the media furore that followed it prompted many parents to decide against having their children vaccinated with the three-in-one jab.
MMR research
Mumps, measles and rubella are all serious diseases, particularly measles which can be fatal. Many doctors were concerned that a drop in vaccination levels could leave many children at risk.
What do other experts say?
The UK government and the vast majority of scientists insist that the three-in-one jab is safe.
Complications of measles
There have been many studies examining the safety of MMR since 1998. All have concluded that the three-in-one jab is safe.
Last year The Lancet, which published the controversial MMR paper in the first place, publicly announced it should never have printed it.
It says the study was flawed and that Dr Andrew Wakefield, the lead author, had a serious conflict of interest.
Complications of mumps
Dr Wakefield insists he has done nothing wrong and that the science behind his study is still valid.
What is the GMC case about?
It is easy to forget that the hearing is not about whether MMR is safe. Instead, it is about the professional conduct of Dr Wakefield and two of his colleagues.
The GMC disciplinary panel will examine whether the doctors failed to get proper ethical approval for their research, and then carried out procedures on children that had not been sanctioned by the ethics committee.
It is also alleged that some of the children who took part did not qualify for the study on the basis of their behavioural symptoms.
The doctors are also accused of failing to tell The Lancet how they recruited patients - leading to inaccuracies in the published research - and Dr Wakefield faces a charge to do with a potential conflict of interest.
Dr Wakefield denies any wrongdoing.
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