Effect of IGFBP-3 on ATRA-induced NBT reduction of HL-60 and NB4 cells. Cells were cultured for 2 days with IGFBP-3 (5 μg/mL) alone or in combination with ATRA (10^–7 M), and differentiation was determined by NBT reduction. Data represent the mean ± SD of triplicate cultures.