The function of CYP27C1 (cytochrome P450 family 27 subfamily C member 1, ENSG00000186684) is as follows. [Isoform 2]: A cytochrome P450 monooxygenase that catalyzes the 3,4 desaturation of all-trans-retinol (also called vitamin A1) to all-trans-3,4-didehydroretinol (also called vitamin A2) in the skin. Desaturates with lower efficiency all-trans retinal and all-trans retinoic acid. Forms minor amounts of 3-hydroxy and 4-hydroxy all- trans-retinol derivatives. Mechanistically, uses molecular oxygen inserting one oxygen atom into a substrate and reducing the second into a water molecule. Two electrons are provided by NADPH via a two-protein mitochondrial transfer system comprising flavoprotein FDXR (adrenodoxin/ferredoxin reductase) and nonheme iron-sulfur protein FDX1 or FDX2 (adrenodoxin/ferredoxin). {ECO:0000269|PubMed:27059013, ECO:0000269|PubMed:28701464}.