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:15610069, PubMed:15907797, PubMed:22745828). Gives rise to a mixture of 16:1 and 18:1 unsaturated fatty acids (PubMed:15610069, PubMed:15907797). Involved in neuronal cell proliferation and differentiation through down-regulation of EGFR/AKT/MAPK and Wnt signaling pathways (PubMed:22745828). 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. May self-associate and form homodimers. Detected in fetal brain, and at lower levels in fetal kidney. Detected in adult brain and pancreas, and at lower levels in kidney and lung. Expressed in spiral ganglion cells and the organ of Corti of fetal cochlea (PubMed:31972369). The histidine box domains are involved in binding the catalytic metal ions. The disease may be caused by variants affecting the gene represented in this entry. This protein has no ortholog in rodents. Belongs to the fatty acid desaturase type 1 family.