Is there anything left to "sex" that is not the "gender construction of biology"? With the fruit fly, this article seeks to rethink the substance of sexual difference--here called "naked sex"--as that part of sex which rebuffs the gender microscope. Naked sex haunts the terms by which feminist scholars have deconstructed the (un)scientific construction of sex, and have challenged biological determinism and gender biases in science. The sex-determining genes appear as the minimal bundle for this returning residue which secures the epistemology of gender that is naked sex. The experimental history (1976-1979) of the Drosophila sex-determining gene, Sex-lethal, displaces the insistent question of "what is real/biological about biological sex" toward an inquiry into the "realization" of sex through sexing and unsexing research practices, wherein sex becomes scientifically performative. Unexpectedly, sex never emerges as naked sex throughout this singular exploration of "How to Do Scientific Things with Sex" in the lab.