Catalyzes C14-demethylation of lanosterol which is critical for ergosterol biosynthesis. It transforms lanosterol into 4,4'-dimethyl cholesta-8,14,24-triene-3-beta-ol (By similarity). Favors C4 dimethylated substrates, the substrate preference order is 24-methylenedihydrolanosterol > 24,25-dihydrolanosterol > lanosterol > obtusifoliol > norlanosterol. a 14alpha-methyl steroid + 3 O2 + 3 reduced [NADPH--hemoprotein reductase] = a Delta(14) steroid + formate + 4 H(+) + 4 H2O + 3 oxidized [NADPH--hemoprotein reductase] Steroid biosynthesis; zymosterol biosynthesis; zymosterol from lanosterol: step 1/6. Expressed in both the insect and mammalian life-cycle stages. Belongs to the cytochrome P450 family.