Catalyzes the stereospecific oxidation of squalene to (S)-2,3-epoxysqualene, and is considered to be a rate-limiting enzyme in steroid biosynthesis. 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. A number of isoforms are produced. According to EST sequences. Expressed in seedlings, leaves, stems and inflorescences. Detected in siliques. SEQ4 or SEQ5 are unable to complement seq1 mutants. Belongs to the squalene monooxygenase family.