Which marker is commonly positive in GIST?

• A. c-kit (CD117)
• B. CD34
• C. S100
• D. CK7

Answer: (B)

GISTs have an immunohistochemical profile that reflects their origin from interstitial cells of Cajal. The most consistent feature is frequent positivity for c-KIT (CD117) and DOG1, which are the backbone markers used to diagnose GIST. CD34 is also commonly positive in many GISTs, making it a helpful supportive marker—positivity in a substantial majority of cases (roughly around 60–70%). S100 and CK7 are usually negative, which helps distinguish GIST from other tumors such as neural tumors (which often show S100 positivity) or carcinomas (which may express CK7). So, the presence of CD34 supports the diagnosis, especially when combined with c-KIT/DOG1 positivity and negativity for S100 and CK7.