BCR and chronic myelogenous leukemia, BCR-ABL1 positive: The clinical resistance to imatinib in CML is caused by two major mechanisms: an amplification of the Bcr–Abl gene or mutations in the catalytic domain of the protein (Gorre et al, 2001; Hochhaus et al, 2002; Roumiantsev et al, 2002; Azam et al, 2003).