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. Expressed mainly in seedlings and inflorescences. SEQ4 or SEQ5 are unable to complement seq1 mutants. Belongs to the squalene monooxygenase family.