GIST staining commonly includes which marker?

• A. c-kit (CD117)
• B. S100
• C. Melan-A
• D. CK7

Answer: (A)

GISTs characteristically express the KIT receptor, so immunostaining for CD117 is commonly positive. This marker reflects the activating KIT mutations that drive many GISTs, making CD117 a primary tool to confirm the diagnosis on biopsy. While other markers like DOG1 and CD34 can also be positive in many GISTs, CD117 is the classic marker most tests rely on to identify these tumors. The other options are not typical for GIST: S100 points toward neural/nerve sheath or melanocytic tumors, Melan-A is a melanocytic marker, and CK7 is an epithelial marker. In practice, CD117 positivity helps distinguish GIST from other mesenchymal tumors of the GI tract and guides targeted therapy with KIT inhibitors.