Rules requiring all athletes in the female category of world ranking events to take a one-time gene test come into force this week.
World Athletics says the sex screening - which detects the presence of a Y chromosome - is to protect the integrity of women's competition.
But how does the latest attempt to tackle one of sport's most contentious issues work? How did we get here, are there concerns, and what are the implications for the debate around gender eligibility? BBC Sport answers the key questions.
What is the test looking for?
The test detects the SRY gene - or 'sex-determining region Y gene' - which is part of the Y chromosome and causes male characteristics to develop.
If a human embryo has XY chromosomes the SRY gene leads to the formation of testes, which then produce hormones including testosterone that lead to male development - increasing muscle mass and strength.
Research has shown athletes who were born male and passed through male puberty have physiological advantages over those born female (with XX chromosomes and no SRY gene).
The test is designed to determine biological sex in cases of athletes who are DSD - those born with 'differences in sex development'. This is a term for a group of rare conditions, whereby a person's hormones, genes and/or reproductive organs may be a mix of male and female characteristics. Some can be born with external female genitalia but functioning testes, and are often certified as female at birth and raised as such.
How will the SRY test be conducted?
It is a cheek swab or blood test, which is done once in an athlete's life by each national federation. If the test is negative for the Y chromosome - ie it is absent - the athlete is eligible to compete in the female category.
However, World Athletics says that a positive test may occur if the athlete has the DSD condition 46 XY, and that "such individuals with typically male chromosomes may have atypical development of reproductive or sexual anatomy due to variations in the SRY gene or other related genetic factors", with some assigned a female gender at birth.
In such cases, unless the athlete has complete androgen insensitivity syndrome (CAIS) - and has not gone through male puberty - they would be ineligible to compete in the female category.
World Athletics is also allowing "a very small number of known DSD athletes to compete under the current regulations", as long as they continue to comply.
How many tests have been done?
World Athletics says more than 90% of the athletes due to take part in the World Championships later this month have been tested, with any outstanding cases to be conducted in Tokyo.
Its president Lord Coe has admitted the time frame has been "tight", and that there has been "added complexity" because genetic testing for non-medical reasons is outlawed in some countries, such as France and Norway, so some athletes have had to give samples overseas.
A group of Canadian athletes also had to be retested after a reported 'test-tube error' meant their cheek swabs failed to comply with requirements.
Why has World Athletics chosen to do this now?
Gender in sport has become more of a major issue since 2009, when South African Caster Semenya won the 800m final at the World Championships in Berlin - going on to dominate at the distance.
She was born with 46 XY 5-ARD (5-alpha-reductase deficiency). People with this particular DSD have male XY chromosomes, but some are recorded as female at birth.
In 2019 it was revealed that World Athletics had claimed in court that Semenya was "biologically male", external, which the runner said she felt insulted by.
Semenya was legally registered as female at birth and has said that although she was born without a uterus and with internal testicles, she has a vagina and is a woman.
