Several aldehyde dehydrogenases catalyse the irreversible oxidation of retinal to retinoic acid. ALDH1A1, 2 and 3 utilise NAD+ as cofactor to oxidise all-trans-retinal (atRAL) to all-trans-retinoic acid (atRA). They are cytosolic enzymes which are functional in their homotetrameric states (Yoshida et al. 1993, Duester 2008, Bchini et al. 2013).