KIT and chronic myelogenous leukemia, BCR-ABL1 positive: Imatinib (IM; formerly STI571) is a tyrosine kinase inhibitor that has strong activity against the BCR/ABL tyrosine kinase, a fusion protein that causes the onset of Philadelphia chromosome positive chronic myeloid leukemia (CML) and broad activity against Src-family tyrosine kinases, such as c-KIT and platelet-derived growth factor receptor, which play a role in gastrointestinal stromal tumors.