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. gleevec blocks the protein made by the bcr/abl oncogene. it is a type of tyrosine kinase inhibitor. also called imatinib mesylate and sti571.