On page 7 of Gender Trouble 1990.
If the immutable character of sex is contested, perhaps this construct called “sex” is as culturally constructed as gender; indeed, perhaps it was always already gender, with the consequence that the distinction between sex and gender turns out to be no distinction at all.
It would make no sense, then, to define gender as the cultural interpretation of sex, if sex itself is a gendered category. Gender ought not to be conceived merely as the cultural inscription of meaning on a pregiven sex (a juridical conception); gender must also designate the very apparatus of production whereby the sexes themselves are established. As a result, gender is not to culture as sex is to nature; gender is also the discursive/cultural means by which “sexed nature” or “a natural sex” is produced and established as “prediscursive,” prior to culture, a politically neutral surface on which culture acts. … At this juncture it is already clear that one way the internal stability and binary frame for sex is effectively secured is by casting the duality of sex in a prediscursive domain.
This production of sex as the prediscursive ought to be understood as the effect of the apparatus of cultural construction designated by gender. How, then, does gender need to be reformulated to encompass the power relations that produce the effect of a prediscursive sex and so conceal that very operation of discursive production?