5,6-trans-Vitamin D3 (5,6-trans-Cholecalciferol;5,6-trans-Colecalciferol) is a photoproduct of vitamin D3[1]. Vitamin D3 is a naturally occuring form of vitamin D. Vitamin D3 induces cell differentiation and prevents proliferation of cancer cells. Normal human keratinocytes in the cultured human keratinocytes by determining their influence on 3H-thymidine incorporation into DNA.

5,6-trans-vitamin D3 only induces significant inhibition at 10(-6) M. The dissociation constants of vitamin D receptor (VDR) for 5,6-trans-vitamin D3 is 560 nM[1].

Reference:
[1]. T C Chen, et al. An evaluation of the biologic activity and vitamin D receptor binding affinity of the photoisomers of vitamin D3 and previtamin D3. J Nutr Biochem. 2000 May;11(5):267-72.