Multiple retinol dehydrogenases (RDH), members of the short-chain dehydrogenase/reductase (SDR) gene family, are candidates for catalysing conversion of retinol into retinal under physiological conditions (Napoli 2012). These include RDH16 (aka RoDH-4, RDH-E, Rdh1), RDH10, DHRS9 (aka retSDR8) and RDHE2. Two of these, Rdh1 (human ortholog RDH16) and Rdh10 (human ortholog RDH10), have been knocked out in mice and display RA-associated phenotypes. Both are membrane bound oxidoreductases that reversibly catalyse the first and rate limiting step in retinoic acid biosynthesis, and use NAD+ as cofactor to the corresponding aldehyde all trans retinal (atRAL) (Gough et al. 1998, Jurukovski et al. 1999, Pecozzi et al. 2003). The other RDH are currently under study, but have not been ablated in mice.