So, caster who was born female...but that's sort of the problem.
It was assumed Caster was female at birth because of an absence of obvious genitalia. She actually grew up with internal testes which gave her the masculine boost of testosterone making her appearance and physical capabilities closer matched to a male competitor than female.
It's utterly unfair that her body and privacy is under such intense scrutiny and highlights why all sporting bodies need to move to a solid binary system that is based on biology. The absence of Y chromosome and normal hormone tolerance levels means you compete as a woman. Anyone else competes in an 'open' category.
It's still binary and makes the playing field obvious and fair. So a 6ft woman with a cracking stride would still have a natural advantage over a 5ft 5 woman in a 100m race...but crucially there's no dubious other advantage that calls into question the validity of the 6ft woman's race results thereby undermining her hard work to get there in the first place.
I wholeheartedly agree with the ruling but really do feel for Caster. Despite her biology she was raised and trained as a woman and this must feel like a vendetta against her, when in truth it's about making it possible for all future athletes to be free of this media circus if they achieve something incredible. A simple and non-invasive blood test to confirm your competitive category and you're done.