Very, very, iirc, but likely one of the reasons women in 40k aren't space marines is that they are genetically enhanced, as well as have multiple organs implanted. I would assume the whole gene splicing piece is probably the hangup.
Again iirc, they are pseudoclones of their primarchs who are pseudoclones of the big E.
I would assume a trans woman could be a space marine then? They probably would never know they were trans though, the life of males that eventually become space marines aren't exactly ones with enough space for that kinda introspection just a constant struggle to survive in some of the most hellish places in the Imperium.
There are also the Adapteus Sororitas, plenty of women in the Imperial Guard, and honestly brutally bad ass women all over the 40k universe.
It's also the name of a tool for working with braided rope.