(a) Architectural variants of class I STCs. α, β, γ domains are shown in red, light blue, and marine blue, respectively. No structure of class II STCs was known before this study. (b) Structurally characterized class II triterpene and diterpene cyclases. SHC is a squalene-hopene cyclase. AtCPS and PtmT2 represent the plant and bacterial ent-copalyl diphosphate synthases, respectively. (c) Overview of the discovery and structure of a class II STC, SsDMS, in this study. SsDMS catalyzes farnesyl diphosphate into drimenyl diphosphate. Apo-SsDMS owns a βγ-didomain architecture that resembles other class II TCs. A Mg2+ binding site was seen in class II TCs for the first time.