The X retinoid receptor agonist 9-cis-retinoic acid and the 24-hydroxylase inhibitor ketoconazole increase activity of 1,25-dihydroxyvitamin d3 in human skin in vivo. - Kang,S.; Li,XY.; Duell,EA.; Voorhees,JJ. (J Invest Dermatol, 1997)