Site-saturated substitution in Saccharomyces cerevisiaeoxidosqualene-lanosterol cyclase at Ile705 position produced three chair-boat-chair (C–B–C) truncated tricyclic compounds, two 17α-exocyclic protosteryl intermediates, two protosteryl C-17 truncated rearranged intermediates and the normal biosynthetic product, lanosterol. These results indicated the importance of the Ile705 residue in affecting lanosterol's C/D ring stabilization including 6-6-5 tricyclic and protosteryl C-17 cations and 17α/β–exocyclic side chain stereochemistry.