Endogenous retinoic acids (RA) which play a role in gene regulation exist as either cis or trans isomers. While CYP26C1 can also hydroxylate the trans form, it is unique in 4-hydroxylating the 9-cis isomer of RA (9cRA) in vitro (Taimi et al. 2004). However, the importance of 9cRA as an endogenous retinoid outside of the pancreas has not been demonstrated.
retinoic acid 4-hydroxylase activity of CYP26C1 [endoplasmic reticulum membrane]