The WNBA CBA states only women can play, but the league won't define the term or explain how eligibility is determined for transgender athletes.
https://www.foxnews.com/outkick-sports/wnba-silent-caitlin-clark-says-leagues-should-determine-who-included-womens-sports
https://www.foxnews.com/outkick-sports/wnba-silent-caitlin-clark-says-leagues-should-determine-who-included-womens-sports