N-terminal 6His-tagged recombinant, human Abl, amino acids 27-end containing the H396P mutation. The Abl tyrosine kinase inhibitor STI571 (Gleevec™) is an effective therapy for stable-phase chronic myeloid leukemia patients. Many patients responding to STI571 later relapse, due to a reactivation of Bcr-Abl activity. In certain cases this appears to correlate with the presence of the H396P mutation, which confers resistance to drug binding due to steric interference. (Corbin AS. et al, Blood, (2002); 101:4611-4614 and La Rosee P. et al, Cancer Research; 62:7149-7153).