In insects, the steroid hormone ecdysone controls the timing of moulting and metamorphosis. Some enzymes involved in ecdysone biosynthesis from dietary cholesterol have been identified but not those for the conversion of 7-dehydrocholesterol to 5β-ketodiol, the so-called ‘Black Box’. Now, Ryusuke Niwa, Tetsuro Shinoda and co-workers report that non-molting glossy (nm-g)/shroud (sro) encodes a short-chain dehydrogenase/reductase that functions in this ecdysteroid biosynthesis Black Box (see p. 1991). The researchers first isolated nm-g by positionally cloning the silkworm nm-g mutant gene. This mutant has low ecdysteroid levels and developmentally arrests as a larva. Next, they discovered that the Drosophila mutant sro, which is a Drosophila Halloween-class embryonic lethal mutant, is caused by the loss of function of an nm-g orthologue. Finally, they report that the application of ecdysteroids or 5β-ketodiol, but not that of cholesterol or 7-dehydrocholesterol, reverses nm-g/sro mutant phenotypes. Thus, the researchers conclude, Nm-g/Sro is an essential, but as yet uncharacterised, component of the ecdysteroid biosynthesis Black Box.