A drug used to treat certain types of leukemia and other blood cancers. It is also used to treat gastrointestinal stromal tumors (GIST), skin tumors called dermatofibrosarcoma protuberans, and a rare condition called systemic mastocytosis. It is also being studied in the treatment of other types of cancer. Imatinib mesylate blocks the protein made by the BCR-ABL oncogene, which may help keep cancer cells from growing and may kill them. Imatinib mesylate is a type of tyrosine kinase inhibitor. Also called Gleevec and STI571.