Catalyzes the reduction of the delta-24 double bond of sterol intermediates during cholesterol biosynthesis (PubMed:11519011, PubMed:21671375, PubMed:25637936, PubMed:22178193). In addition to its cholesterol-synthesizing activity, can protect cells from oxidative stress by reducing caspase 3 activity during apoptosis induced by oxidative stress (PubMed:11007892, PubMed:22010141). Also protects against amyloid-beta peptide-induced apoptosis (PubMed:11007892). cholesterol + NADP(+) = desmosterol + H(+) + NADPH H(+) + lanosterol + NADPH = 24,25-dihydrolanosterol + NADP(+) 5alpha-cholest-8-en-3beta-ol + NADP(+) = H(+) + NADPH + zymosterol Steroid biosynthesis; cholesterol biosynthesis. Interacts with DHCR7; this interaction regulates DHCR7 activity. Highly expressed in brain and adrenal gland with moderate expression in liver, lung, spleen, prostate and spinal cord. Low expression in heart, uterus and prostate. Undetectable in blood cells. In the brain, strongly expressed in cortical regions, substantia nigra, caudate nucleus, hippocampus, medulla oblongata and pons. In brains affected by Alzheimer disease, expression in the inferior temporal lobe is substantially lower than in the frontal cortex. The disease is caused by variants affecting the gene represented in this entry. Belongs to the FAD-binding oxidoreductase/transferase type 4 family. Extended N-terminus.