Inhibition of cell proliferation by 20S(OH)D3, 20S,25(OH)2D3, 20S,23S(OH)2D3 and 20S,23R(OH)2D3 in comparison to 1α,25(OH)2D3. Human epidermal (HaCaT) keratinocytes (A, C) or dermal fibroblasts (B, D) were treated for 24 h (1) or 48 h (2) with graded concentrations of the compounds: 0.1, 1, 10, or 100 nM. Ethanol (0.1%) served as a negative control. Data are means ± SD (n = 6) and were analyzed using Student’s t-test with p < 0.05 (*), p < 0.01 (**) or p < 0.001 (***).