Effect of IGFBP-3 on ATRA- or 9-cis RA–induced growth inhibition of NB4 cells. Cells were cultured with ATRA (10^–7 M) or 9-cis RA (10^–7 M) alone or in combination with IGFBP-3 (5 μg/mL). Data represent the mean ± SD of triplicate cultures, and results represent at least 3 independent experiments.