The remarkable cyclization mechanism of the formation of the 6-6-6-5 tetracyclic lanosterol (a key triterpenoid intermediate in the biosynthesis of cholesterol) from the acyclic 2,3-oxidosqualene catalyzedb yo xidosqualenec yclase (OSC) has stimulated the interest of chemists and biologists for over ahalf century.Herein, the elaborate,state-of-the-art twodimensional (2D) QM/MM MD simulations have clearly shown that the cyclization of the A-C rings involves an early concerted, but highly asynchronous cyclization, to yield as table intermediate with "6-6-5" rings followed by the ring expansion of the C-ring concomitant with the formation of the D-ring to yield the "6-6-6-5" protosterol cation. The calculated reaction barrier of the rate-limiting step ( % 22 kcal mol À1 )i s comparable to the experimental kinetic results.Furthermore all previous experimental mutagenic evidence is highly consistent with the identified reaction mechanism.