CD117 is a cell membrane protein encoded by the c-kit proto-oncogene. CD117 is expressed in mast cells, skin melanocytes and interstitial Cajal cells (ICC). These cells show a strong membrane and cytoplasmic staining. CD117 is also expressed in various epithelia (salivary glands, renal tubular cells etc.). Appendix serves as a good positive and negative control tissue. Neoplasms such as gastrointestinal stromal tumor (GIST), mast cell neoplasms and many other (seminoma, Mercel cell carcinoma etc.) express CD117. This antibody (together with DOG-1, CD34, SMA) is of great importance in the diagnosis of GIST, because of specific treatment of GIST patient with Gleveec.