Extent of cellular resistance for mutations identified within the Bcr-Abl kinase domain to imatinib and PD166326. Ribbon representation of the c-Abl kinase domain in complex with imatinib (A; derived from Protein Data Bank 1IEP,³⁴  with the A-loop in a closed conformation) and the pyrido-pyrimidine PD173955 (B; PDB 1M52,¹⁵  with the A-loop in an open conformation), with C-helix in light green, P-loop in dusky pink, and A-loop in magenta. Labels indicate the residue number of human c-Abl kinase type Ia. The colors of the spheres represent the degree of cellular resistance to imatinib (A) and PD166326 (B), expressed as fold cellular IC50 of wild-type Bcr-Abl in Ba/F3 cells.