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 (PubMed:20149798, PubMed:8619637). 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) (PubMed:20149798, PubMed:8619637). 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. Ubiquitously expressed with highest levels in testis, ovary, adrenal, prostate, liver, kidney and lung. Belongs to the cytochrome P450 family. It is uncertain whether Met-1 or Met-7 is the initiator. Truncated N-terminus.