The recent ruling by a French court that a 64-year-old individual who describes himself as "intersex" can record his gender as "neutral" on official forms has been hailed by LGBT campaigners as a major victory.
"For 64 years I've lived with both sexes,” the person, who wants to remain anonymous, said. “I finally have the feeling of being recognised for what I am."
But the case, which sets a European precedent, has prompted warnings it could lead to a spate of claims from people demanding recognition of a "third" sex.
1. What does being "intersex" mean?
According to the UK Intersex Association, intersex people are not born with a sexual anatomy or biology that can be typically described as male or female.
For example, genitals might seem ambiguous to doctors at birth, or they may become so at puberty.
In addition, an individual may appear female on the outside but be anatomically male inside - or vice versa.
Hormones may not be produced or absorbed in the same way, and an intersex person will not usually be fertile. Their chromosomes might be both XX and XY - genetically male and female.
The word also works in its own right as an identity, and has been reclaimed by the intersex community in order to try and gain recognition for what is often one of the least visible gender and sexual identity minorities. A person who says they are intersex may identify as female, male, both, neither or as intersex.
2. How has the medical profession treated intersexuality?
Intersexuality is a Disorder of Sex Development (DSD), according to the NHS website, which recommends assigning a gender to a child at birth.
"Your care team may advise you to delay registering your child's birth for a few days while these tests are carried out," the advice says.
"Afterwards, the results will be explained to you and you will have a discussion with specialists about whether to bring your baby up as a boy or a girl."
However, the UK Intersex Association warns strongly against the binary idea of male and female, which it says can make being intersex so difficult.
"If gender role is imposed externally by pressure from others, such as parents or those with vested interests, the conflict for the individual may be highly destructive," the association says.
Sarah Graham, a stand-up comic, therapist and activist, says she was made to take hormone treatment from the age of 12, which she has now changed. She says about 90% of children who are taken for "corrective" surgery come out of the operating theatre female, because this is easier to "create".
3. Who are intersex people I can find out about?
According to the Intersex Society of North America, one in 1,500 babies are born as intersex, though it adds that smaller sexual differentiations are often not noticed until later. As such, most people can expect to have met at least one intersex person in their life.
Prominent members of the intersex community include:
Georgina Somerset, the UK's first openly intersex woman who had been brought up as a boy, struggled to be recognised as a woman and was legally married as female in church after officially changing sex.
Sarah Graham is a therapist, intersex activist and has been described as the UK's "national LGBT treasure".
Lady Colin Campbell, is a British writer and biographer of Princess Diana, refers to herself as intersex and was brought up as a boy until her teens.
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