A cytochrome P450 monooxygenase involved in retinoid metabolism. Hydroxylates all trans-retinoic acid (atRA) to 4- hydroxyretinoate and may modulate atRA signaling and clearance. Mechanistically, uses molecular oxygen inserting one oxygen atom into a substrate, and reducing the second into a water molecule, with two electrons provided by NADPH via cytochrome P450 reductase (CPR; NADPH- ferrihemoprotein reductase). . This is the function of CYP2C18 (cytochrome P450 family 2 subfamily C member 18, ENSG00000108242).