CD117 (C-kit) is a transmembrane glycoprotein receptor tyrosine kinase and is encoded by the KIT gene. It plays an important role during the development of germ cells, mast cells, hematopoietic stem cells, melanocytes, and interstitial cells of Cajal. Activating mutations in this gene are associated with gastrointestinal stromal tumors, testicular seminoma, melanoma, mast cell disease, and acute myeloid leukemia. With immunohistochemistry, CD117 is detected in almost 80% of seminomas. The staining pattern of C-kit is identical to that of PLAP in IGCN (membrane staining with some cytoplasmic staining).