6Hepatocyte SkinKidneyVDR targets8Spontaneous isomerisationEndocrine modulatorsUV B radiation from sunlightVitamin D from DietCYP27A1Vitamin D binding proteinInactive24,25-OH-vitamin DVitamin D binding proteinVDRDHCR7CYP24A1CalcitriolCalcidiolCYP27B1RXRBCholesterolParathyroid hormonePiRXRACYP2R1 CalcidiolCholecalciferol7-Dehydro-cholesterolPrevitamin D3Caalpha-globulin41, 2, 7Activation5Tissue-specific regulation