K562^R and AR230^R cells are resistant to imatinib despite inhibition of BCR-ABL1 kinase. (A) Whole cell extracts were resolved by sodium dodecyl sulfate–polyacrylamide gel electrophoresis (SDS-PAGE) and probed with antibodies directed against ABL1 and phospho-ABL1; α-tubulin was analyzed as a control. (B) Bar graphs represent imatinib IC₅₀ for K562^S, K562^R, AR230^S, and AR230^R cells measured by treating these cells with increasing concentrations of imatinib and quantifying cell proliferation by MTS assay after 72 hours. Errors bars represent standard error of the mean (SEM); *P < .05.