Stearoyl-CoA desaturase that utilizes O(2) and electrons from reduced cytochrome b5 to introduce the first double bond into saturated fatty acyl-CoA substrates (PubMed:16443825). Catalyzes the insertion of a cis double bond at the delta-9 position into fatty acyl-CoA substrates including palmitoyl-CoA and stearoyl-CoA (PubMed:16443825). Gives rise to a mixture of 16:1 and 18:1 unsaturated fatty acids (PubMed:16443825). Contributes to the biosynthesis of membrane phospholipids, cholesterol esters and triglycerides, especially during embryonic development and in neonates (PubMed:16118274). Important for normal permeability barrier function of the skin in neonates (PubMed:16118274). 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 + octadecanoyl-CoA = (9Z)-octadecenoyl-CoA + 2 Fe(III)-[cytochrome b5] + 2 H2O 2 Fe(II)-[cytochrome b5] + 2 H(+) + hexadecanoyl-CoA + O2 = (9Z)-hexadecenoyl-CoA + 2 Fe(III)-[cytochrome b5] + 2 H2O Expected to bind 2 Fe(2+) ions per subunit. Detected in brain and skin (PubMed:10545940, PubMed:11161812, PubMed:16118274). Highly expressed in brain, and detected at low levels in heart, stomach, lung and testis (PubMed:11161812, PubMed:12815040). Detected both in dermis and epidermis (PubMed:16118274). Highly expressed during embryonic development and during the first three weeks after birth. Expression is low in adults. Up-regulated by agonists that activate NR1H3 (PubMed:12815040). Slightly down-regulated by a high-carbohydrate diet enriched in unsaturated fatty acids (PubMed:12815040). The histidine box domains are involved in binding the catalytic metal ions. Mutant mice are born at the expected Mendelian rate. Neonates are smaller than wild-type and present high mortality, ranging from 70 to 100%, depending on the genetic background. Neonates display a shiny skin, but after a few hours their skin appears dry and cracked. The permeability barrier function of their skin is impaired, leading to rapid weight loss due to dehydration. Their epidermis has decreased levels of cholesterol esters, triglycerides, acylceramide, and glucosylacylceramide containing unsaturated fatty acids. In mutant neonates, triglyceride levels in liver and blood plasma are reduced by half, due to strongly reduced levels of stearoyl-CoA desaturase activity in the liver and strongly reduced levels of triglyceride biosynthesis. In contrast, the levels of stearoyl-CoA desaturase activity are normal in adult mice deficient for Scd2. Adult mice display kinked tails. Belongs to the fatty acid desaturase type 1 family.