CD117 (cKit) is a transmembrane receptor tyrosine kinase. It is expressed in many tissues and cells, such as tissue mast cells, skin basal cells, melanocytes, breast glandular epithelial cells, dermal sweat gland, esophageal glands, testicular and ovarian interstitial cells. Abnormal expression of cKit has been implicated in pathogenesis of myeloid leukemias. cKit expression has also been demonstrated in solid tumors including gastrointestinal stromal tumor (GIST), melanomas, breast carcinomas and small cell lung carcinoma. C-Kit pharmDXTM is indicated as an aid in the differential diagnosis of GIST. Accurate assessment of CD117 protein expression using cKIT testing is a critical factor in the diagnosis of GIST and is becoming increasingly important in clinical management, including the use of imatinib mesylate (Gleevec®) therapy.