A cytochrome P450 monooxygenase involved in sterol biosynthesis. Catalyzes 14-alpha demethylation of lanosterol and 24,25-dihydrolanosterol likely through sequential oxidative conversion of 14-alpha methyl group to hydroxymethyl, then to carboxylaldehyde, followed by the formation of the delta 14,15 double bond in the sterol core and concomitant release of formic acid. Mechanistically, uses molecular oxygen inserting one oxygen atom into a substrate, and reducing the second into a water molecule, with two electrons provided by NADPH via cytochrome P450 reductase (CPR; NADPH-ferrihemoprotein reductase). 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] lanosterol + 3 O2 + 3 reduced [NADPH--hemoprotein reductase] = 4,4-dimethyl-5alpha-cholesta-8,14,24-trien-3beta-ol + formate + 4 H(+) + 4 H2O + 3 oxidized [NADPH--hemoprotein reductase] 24,25-dihydrolanosterol + 3 O2 + 3 reduced [NADPH--hemoprotein reductase] = 4,4-dimethyl-8,14-cholestadien-3beta-ol + formate + 4 H(+) + 4 H2O + 3 oxidized [NADPH--hemoprotein reductase] Inhibited by azalanstat. Inhibited by azole antifungal agents ketoconazole, itraconazole and fluconazole. Steroid biosynthesis; zymosterol biosynthesis; zymosterol from lanosterol: step 1/6. Belongs to the cytochrome P450 family. It is uncertain whether Met-1 or Met-7 is the initiator.