A drug used to treat different types of leukemia and other cancers of the blood, gastrointestinal stromal tumors, 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. It is a type of tyrosine kinase inhibitor. Also called Gleevec and STI571.