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 and stearoyl-CoA (PubMed:20228221). Gives rise to a mixture of 16:1 and 18:1 unsaturated fatty acids (PubMed:20228221). Contributes to the biosynthesis of membrane phospholipids, cholesterol esters and triglycerides, especially during embryonic development and in neonates. Important for normal permeability barrier function of the skin in neonates. 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 adipose tissue, and at much lower levels in testis. Detected in liver when rats are kept on a fat-free diet, but not when their food contains unsaturated fatty acids. Up-regulated in liver in the absence of dietary unsaturated fatty acids(PubMed:1982442). Expression in adipose tissue seems to be constitutive (PubMed:1982442). The histidine box domains are involved in binding the catalytic metal ions. Belongs to the fatty acid desaturase type 1 family.