A federal appeals court is to examine a law on Monday banning transgender athletes from competing in girls' and women's sports, an issue which has become highly politicized in the United States.
The 9th US Circuit Court of Appeals in California will hear arguments in a case challenging a law passed last year in the western state of Idaho.
Idaho was the first US state to pass legislation banning transgender athletes from taking part in girls' or women's sports.
The plaintiff in the landmark case is Lindsay Hecox, a 20-year-old transgender student-athlete at Boise State University who is barred by the law from competing on the women's track and field team.
Bills similar to that passed in Idaho are in various stages of the legislative process in around 20 other Republican-ruled states around the country.
Ahead of Monday's appeals court hearing in Pasadena, California, former Olympic champion Caitlyn Jenner added her voice to the debate.
Jenner, who won a gold medal in the men's decathlon at the 1976 Summer Olympics and came out as a woman in 2015, said she was opposed to letting transgender girls take part in girls' sports.
The 71-year-old Jenner, who is currently running for governor of California, said it was a "question of fairness."
"That's why I oppose biological boys who are trans competing in girls' sports in school," Jenner told TMZ. "It just isn't fair. And we have to protect girls' sports in our schools."
Jenner supported Donald Trump's candidacy in the 2016 presidential election and is running as a Republican for governor in the state's recall election.
- 'I am one of them' -
The LGBTQ group Equality California denounced Jenner's comments as political posturing.
"Here are the facts: @Caitlyn_Jenner is willing to sacrifice the health & well-being of #trans kids to win votes," it said.
The "fairness" argument advanced by Jenner has been the one used in Idaho and other states and it has fractured the country along political lines.
Chase Strangio, a lawyer for the civil rights group the ACLU, which is fighting against the ban, said it is part of a "nonstop attack on trans kids."
"This is very much a solution in search of a problem," Strangio said. "We have trans kids already experiencing so much discrimination."
A district court blocked the Idaho bill sending it to the appeals court in California.
"It appears transgender women have not and could not 'displace' cisgender women in athletics 'to a substantial extent,'" said Judge David Nye.
"Although the ratio of males to females is roughly one to one, less than one percent of the population is transgender," Nye said. "Presumably, this means approximately one half of one percent of the population is made up of transgender females."
The Trump administration had supported the Idaho ban but it is being opposed by the government of Democrat Joe Biden.
A coalition of athletes has rallied behind Hecox, the student at Boise State University who is the plaintiff in the case.
They include former tennis champion Billie Jean King, Megan Rapinoe of the World Cup-winning women's soccer team and WNBA star Candace Parker.
"What I want to do is just run, have a team, have friends on the team just supporting me," Hecox said in a video released by the ACLU. "I am not trying to take away girls' scholarships or trophies or places.
"I just want to be one of them. I am one of them."