Induction of drug resistance by elevated HR activity. (A) ARP myeloma cells were cultured in the presence of dexamethasone (10⁻⁸ mol/L) alone or dex (10⁻⁸ mol/L) and nickel chloride (0.3 mg/mL). Live cell number was determined weekly by trypan blue staining. After 2 weeks of culture, nickel was removed from the dexamethasone and nickel-treated cultures. The figure is representative of 3 independent experiments. (B) Dexamethasone-resistant MM cells obtained in panel A were cultured in higher concentrations of dex (10⁻⁶ mol/L). Live cell number of control and dexamethasone-resistant ARP cells was determined by trypan blue staining.