DHCR7 and Cognitive impairment: Homozygous and heterozygous mutations in human DHCR7 lead to the occurrence of Smith-Lemli-Opitz syndrome (SLOS), which is manifested by elevated levels of the 7-dehydrocholesterol and little cholesterol synthesis in patients, and symptoms such as limb defects, genital abnormalities, and cognitive impairment (Malgorzata and Nowaczyk, 1998).