VDR and lung cancer: Afterwards, we analyzed the expression of the following key proteins that regulate the activity of vitamin D: VDR, CYP24A1 (24-hydroxylase, the enzyme responsible for vitamin D deactivation and the strongest known vitamin D-responsive gene), and RXRα (retinoid X receptor α, which together with VDR forms a heterodimer binding, e.g., to the promoter sequence of the CYP24A1) [31], to check whether their expression was modulated by RESV and PRI-2191 in lung cancer cells.