CD 117 (KIT) is a transmembrane, tyrosine kinase growth factor receptor which is expressed on numerous diverse fetal and adult cells including hematopoietic cells, mast cells, melanocytes, germ cells, and the interstitial cells of Cajal. Its expression in tumors is also diverse, but with selective use and attention to specific staining patterns, this marker is useful in the identification of gastrointestinal stromal tumors, mast cell tumors, and seminomatous germ cell tumors.