Component of the triterpene saponins (e.g. ginsenosides or panaxosides) and phytosterols biosynthetic pathways (PubMed:19857882, PubMed:29378087). Catalyzes the first oxygenation step in sterol biosynthesis and is suggested to be one of the rate-limiting enzymes in this pathway (By similarity). O2 + reduced [NADPH--hemoprotein reductase] + squalene = (S)-2,3-epoxysqualene + H(+) + H2O + oxidized [NADPH--hemoprotein reductase] Terpene metabolism; lanosterol biosynthesis; lanosterol from farnesyl diphosphate: step 2/3. Weak expression in petioles and flower buds and barely detectable in roots and leaves. In petioles, preferentially observed in vascular bundle tissue (phloem cells and parenchymatous cells near xylem) and resin ducts. Repressed by methyl jasmonate (MeJA) (PubMed:19857882). Accumulates in the presence of squalene (PubMed:19857882). Belongs to the squalene monooxygenase family.