Effects of the combination of TT or 9CTT with VD₃ on the NBT-reducing activity of APL-derived NB4 (A) and HT93 cells (B) and non-APL-derived HL-60 (C) and U937 cells (D). Cells (5 × 10⁴ cells/mL) were treated with VD₃ in the absence (•) or presence of 3 × 10⁻⁶ mol/L TT (▴) or 9CTT (▪) for 4 days. Values represent the means ± SD of three separate experiments. *P < .05, where the enhancing effects of 9CTT on VD₃ were compared with those of TT.