Catalyzes the stereospecific oxidation of squalene to (S)-2,3-epoxysqualene, and is considered to be a rate-limiting enzyme in steroid biosynthesis. Can produce not only oxidosqualene, but also 2,3:22,23-dioxidosqualene. Main squalene epoxidase in the root. Sqe1 mutants may show defects in membrane lipid rafts, impairing the correct localization of RHD2 NADPH oxidase and the proper polarized production of ROS. 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. Expressed in seedlings, leaves, stems, inflorescences, sepals, style and siliques. Expressed in expanded cotyledons, root tips and cortical cells of the root elongation zone, but not in root hair cells. In leaves, expressed in most cells, with a very strong expression in stomata. Non viable, sterile dwarf plants with short, highly branched roots. SEQ4 or SEQ5 are unable to complement seq1 mutants. Belongs to the squalene monooxygenase family.