KIT and gastrointestinal stromal tumor: In gastrointestinal stromal tumors (GISTs), the most common mesenchymal neoplasm of the gastrointestinal tract, mechanisms of resistance to imatinib mesylate (GleevecR) include both de novo and, more frequently, acquired resistance, which may occur after several months of drug administration and most often is based upon an acquired second mutation in the c-kit and PDGFRα genes [18, 19].