In 2009, South African runner Caster Semenya, aged 18 years, won the 800 m at the World Championships in Athletics in Berlin, only to be rapidly declared “a woman, but maybe not 100%” by the general secretary of the International Association of Athletic Federations (IAAF), and barred from competing. After being reinstated, she won Olympic Gold in 2012 and 2016 and became the 800 m World Champion in London in 2017. However, by the time the World Championships in Athletics take place between Sept 28 and Oct 6, 2019, new eligibility regulations for female classification from the IAAF,1 based on the interpretation of data2 from studies of androgen concentrations in elite athletes, might have prevented Semenya from competing again.