The molting hormone of insects. It was isolated in crystalline form from the common silkworm (Bombyx mori) by A. Butenandt and P. Karlson in 1954, and its chemical structure was elucidated in 1965. It is a derivative of cholesterol and its structure can be described as 2β,3β,14α,22R,25-penta-hydroxy-Δ7-5β-cholestene-6-one. Shortly thereafter the molting hormone of crustaceans (crustecdysone) was isolated and identified as 20-hydroxy-ecdysone (Fig. 1). The sample compound was identified as one of the active compounds in B. mori extracts by P. Hocks and coworkers, and H. Hoffmeister and coworkers proposed the name ecdysterone. A group of Japanese chemists, working on the isolation of steroids from plants, came across some similarities of the spectra of 20-hydroxy-ecdysone and their compounds inokosterone and isoinokosterone; the latter was identical with 20-hydroxy-ecdysone. Inokosterone carries the last hydroxy group in position 26 instead of 25, which reduces the biological activity by a factor of 10. Several other structurally related and biologically active steroids have been isolated from plant sources, including ecdysone, but nothing is known about a possible biological function of these steroids in plants. See also: Steroid