Female athletes with Differences of Sexual Development, or DSD, are the focus of athletics' governing body. The International Association of Athletics Federations (IAAF) has introduced new eligibility rules they need to abide by if they want to compete in track events between 400m and one mile (1.6km).
The most high-profile athlete affected, two-time 800m Olympic champion Caster Semenya, has been fighting the new regulations in the courts at the Court of Arbitration for Sport (CAS) and the Swiss Federal Tribunal (SFT).
But what does it mean to be a DSD person, and what are the rules in place that Semenya says are discriminatory?
What does 'DSD' mean?
According to the United Kingdom's National Health Service (NHS), DSD "is a group of rare conditions involving genes, hormones and reproductive organs, including genitals. It means a person’s sex development is different to most other people’s."
Simply put, female athletes with the condition naturally produce more testosterone in their bodies than women without the condition.
The IAAF says its rules only apply to individuals who are "androgen-sensitive", meaning they can make use of the androgen and higher testosterone levels in their bodies.
Aside from Semenya, the other two runners on the women's 800m podium at Rio 2016 – silver medallist Francine Niyonsaba of Burundi and bronze medallist Margaret Wambui of Kenya – are both also affected by the new regulations among other athletes.
What do the affected athletes say?
Semenya has argued that the rules are discriminatory and unfair. "I know that the IAAF's regulations have always targeted me specifically," she said through her lawyers when the CAS first ruled against her.
"For a decade the IAAF has tried to slow me down, but this has actually made me stronger," the statement added.
"No human can stop me running," Semenya said defiantly after her last – to date – international 800m race.
The London 2012 and Rio 2016 champion has also received vocal public backing from Athletics South Africa and the South African government.
Francine Niyonsaba, the runner-up behind the South African in the Rio 800m, told the Olympic Channel: "For me, it's about discrimination. It doesn't make sense.
"For sure, I didn't choose to be born like this," she said pointedly.
Asked if the regulations would change anything for her, she said she would consider changing events.
"I can even run the marathon. I will keep my passion. I love running, and I will not stop running."
Kenya's Margaret Nyairera Wambui is another affected athlete. She spoke to the Olympic Channel at the Kenyan World Championship trials, which she attended as a spectator.
"This was my career, this is my talent; this was feeding me," she said. "This was how I was earning my upkeep and suddenly it has been blocked."
She rejected the IAAF's suggestion of medication to control her testosterone levels. "Drugs is out for me. Other options like moving up to the long distance or scaling down, I can consider, but drugs? No.
"I can only ask one thing, those making decisions should consider if I was a family member, a sister. Let them think of it like if it was their sister in this situation, and she’s the sole bread winner of the family, would they still take that decision?
"Sports should be fair. So why not create a rule that will be fair to all? Why stop, or block others?"
What are the regulations and who do they affect?
The IAAF says its DSD Regulations, apply to legally female or intersex athletes who have:
- XY sex chromosomes
- Testes instead of ovaries
- A blood testosterone level "in the male range"
These athletes cannot take part in women's races from 400m to one mile, including combined events (such as the pentathlon, heptathlon, or decathlon) over those distances, at an international competition. Nor can they set world records in these events in non-international competitions where they are allowed to compete.
To be eligible, the IAAF wants individuals to lower their blood testosterone levels to under 5 nmol/L (nanomoles per litre) "because that is the highest level that a healthy woman with ovaries would have", it claims.
Athletes will need to do this for at least six months to become eligible, then maintain that level continuously in and out of competition for future events to remain eligible.
The athletics governing body adds this is not necessary to compete in other female category events, or in men's races.
What does the IAAF say?
When Semenya first took her arguments to the CAS, IAAF President Sebastian Coe said: "The core value for the IAAF is the empowerment of girls and women through athletics. The regulations that we are introducing are there to protect the sanctity of fair and open competition."
The IAAF insists the DSD Regulations are necessary "to ensure fair and meaningful competition within the female classification, for the benefit of the broad class of female athletes.
"The IAAF is convinced there are some contexts, sport being one of them, where biology has to trump identity," it said when the CAS suspended the rules applying to Semenya.
"The IAAF also believes the right to participate in sport does not translate to a right to self-identify into a competition category or an event, or to insist on inclusion in a preferred event, or to win in a particular event, without regard to the legitimate rules of the sport or the criteria for entry," it said.
At a news conference ahead of the 2019 IAAF World Championships, Coe added, "The responsibility we have is to protect that so-called 'level playing field', and we have done that."
Three methods suggested by the governing body for affected athletes to reduce their blood testosterone is to take medication, a monthly injection, or have their testes surgically removed.
"It is their choice whether or not to have any treatment, and (if so) which treatment to have. In particular, the IAAF does not insist on surgery."
International Olympic Committee president Thomas Bach expressed his sympathy for Semenya in May, saying the issue was "extremely delicate and extremely difficult".
Semenya's legal fight
The IAAF first introduced rules to limit the participation of female athletes with high androgen levels in 2011. Known as the Hyperandrogenism Regulations, those rules set the testosterone limit for women with hyperandrogenism at 10 nmol/L (double the limit under the DSD Regulations).
The Hyperandrogenism Regulations were the subject of a legal challenge by Indian sprinter Dutee Chand in 2014. In 2015, the CAS partially upheld Chand's case, which was documented in the Olympic Channel original series Foul Play.
In early 2018, the IAAF withdrew their Hyperandrogenism Regulations and a month later introduced their new DSD Regulations.
The South African took her case to the Swiss Federal Tribunal, who suspended the regulations pending another appeal, leading to her being provisionally named in South Africa's 2019 World Championships squad.
But the Swiss court later reversed its ruling, ending Semenya's hopes of competing in Doha.
Switch to football
With an appearance at the 2019 IAAF World Championships in Doha out of the question, Semenya has been turning her eye to other things.
Semenya is expected to register to play with the team, which counts a number of South African internationals in their squad, for the 2020 season.
But the world of athletics will still be seeing her in the future – she insists she's not given up running just yet.