CD117 is a 145-160 kDa cell membrane protein encoded by the c-kit proto-oncogene. CD117 is required for the development and growth of a large number of cells expressing this protein. The cells (particularly the mast cells) show a strong membrane as well as cytoplasmic staining. CD117 moreover is expressed in various epithelia (breast, sweat glands and salivary glands, renal tubular cells, thyroid follicular cells), usually showing a weaker, cytoplasmic reaction. CD117 is also demonstrated in testicular and ovarian interstitial cells, in neurons of the central nervous system (cerebellum, hippocampus, and dorsal horn of the spinal cord), and in immature myeloid cells. CD117 does not occur in smooth muscle cells. Family of transmembrane proteins essential for the regulation of cell growth and maintenance. When a growth factor is bound to its receptor, the latter is phosphorylated and begins a cascade of intracytoplasmic signals. Cells involved in the generation of electrical pacemaker activity for gastrointestinal motility. The cells are considered to be the origin of gastrointestinal stromal tumours. CD117 is of great importance for the classification of mesenchymal tumours of the gastrointestinal tract (including the mesentery). CD117 may also be used for classification of germinal cell tumours, as seminoma/dysgerminoma stains in the majority of cases, showing a strong membranous staining, while embryonal carcinoma stains in a small proportion of cases, with a weaker, membranous reaction. An panel for distinguishing seminoma from embryonal carcinoma should include CD30. Also in the identification of mast cell neoplasms CD117 has a potential. Appendix is recommended as positive and negative tissue controls for CD117.