Inhibition of Syk tyrosine kinase activity restores nilotinib sensitivity. Proliferation of the nilotinib-sensitive (Ks) and resistant cell lines (K-rn) were tested in MTS assays in the presence of increasing concentrations of nilotinib in the absence (A,C) or the presence (B,D) of R406 (5μM). Results are expressed as the mean of the optical density (OD) of the 4-well set standardized in comparison with the starting optical density at day 0, which is directly proportional to the number of viable cells. K-rn cells were grown in the presence of nilotinib (20nM), R406 (5μM) or a combination of nilotinib plus R406. Apoptosis was detected by flow cytometry using annexin V–FITC at 24, 48, and 72 hours. Results, expressed as the percentage of annexin V positive cells, are from one experiment representative of 3 (E).