Effect of As₂O₃ on NB4 and NB4-As^R cell growth. (A) Exponentially growing cells were seeded at 10⁵ cells/mL and incubated for 4 days without As₂O₃ (NB4 (□), NB4-As^R cells (▪), or with 10⁻⁶ mol/L As₂O₃(NB4-As^R (⧫), NB4 (▵). Each value represents the mean ± standard deviation (SD) of three independent determinations. (B) Apoptotic effect of As₂O₃on NB4 or NB4-As^R cells. NB4 (▵) and NB4-As^R(⧫) were treated for 5 days with the concentrations indicated in the plot. The percentage of apoptosis (%) was determined by a TUNEL assay.