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. Catalyzes the insertion of a cis double bond at the delta-9 position into fatty acyl-CoA substrates including palmitoyl-CoA (PubMed:16443825, PubMed:26098370). Has a strong preference for saturated fatty acids with chain lengths of 14 or 16 carbon atoms (C14:0 and C16:0), and has only very low activity with stearatate (C18:0) (PubMed:16443825, PubMed:26098370). Required for the biosynthesis of membrane phospholipids, cholesterol esters and triglycerides (By similarity). 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 skin, but at lower levels compared to Scd1. Detected in the middlle part of the sebaceous gland, but not in hair follicle. Not detected in liver and brain. Expression is increased during the first eight days (anagen) of the hair cycle in male mice, and is low during the quiescent phase (telogen) of the hair cycle. Expression is very low throughout the hair cycle in female mice. The histidine box domains are involved in binding the catalytic metal ions. Belongs to the fatty acid desaturase type 1 family.