CD117, c-kit is a tyrosine kinase receptor found on interstitial cells of Cajal, germ cells, bone marrow stem cells, melanocytes, breast epithelium and mast cells. This receptor is found on a wide variety of tumor cells (follicular and papillary carcinoma of thyroid, adenocarcinomas from endometrium, lung, ovary, pancreas, breast; malignant melanoma, endodermal sinus tumor, and small cell carcinoma) but has been particularly useful in differentiating gastrointestinal stromal tumors from Kaposi's sarcoma, and tumors of smooth muscle origin.