Catalyzes the reduction of the C14-unsaturated bond of lanosterol, as part of the metabolic pathway leading to cholesterol biosynthesis. 4,4-dimethyl-5alpha-cholesta-8,24-dien-3beta-ol + NADP(+) = 4,4-dimethyl-5alpha-cholesta-8,14,24-trien-3beta-ol + H(+) + NADPH 5alpha-cholest-8,14-dien-3beta-ol + H(+) + NADPH = 5alpha-cholest-8-en-3beta-ol + NADP(+) 4,4-dimethyl-8,14-cholestadien-3beta-ol + H(+) + NADPH = 4,4-dimethyl-5alpha-cholest-8-en-3beta-ol + NADP(+) Steroid biosynthesis; cholesterol biosynthesis. Strongly expressed in liver, weaker in ovary, testis, kidney and brain. Mice develop normally, appear healthy and are fertile. Belongs to the ERG4/ERG24 family.