Enables 7-dehydrocholesterol reductase activity. Involved in cholesterol biosynthetic process and regulation of cholesterol biosynthetic process. Located in intracellular membrane-bounded organelle. Human ortholog(s) of this gene implicated in Smith-Lemli-Opitz syndrome. Orthologous to human DHCR7 (7-dehydrocholesterol reductase). [provided by Alliance of Genome Resources, Apr 2022]