Which staining pattern is described as a known GIST marker combination?

• A. CK7 and CDX2
• B. TTF-1 and PAX8
• C. Melan-A and PAX8
• D. c-kit (CD117) and DOG-1

Answer: (D)

Immunohistochemical staining patterns are used to identify GISTs. Gastrointestinal stromal tumors typically express c-kit (CD117) and DOG-1, which together provide a reliable signature: CD117 is positive in the majority of cases due to KIT mutations, and DOG-1 is a highly sensitive and specific marker for GIST that can help in cases where CD117 alone is inconclusive. Using both markers increases diagnostic confidence and is widely described as the classic GIST immunophenotype. The other options reflect markers associated with different tissue origins—CK7 with CDX2 points toward certain epithelial/intestinal carcinomas, TTF-1 with PAX8 aligns with lung/thyroid and renal/Müllerian lineages, and Melan-A with PAX8 would be more consistent with melanocytic or renal/Müllerian contexts—not with GIST. Therefore, the combination of c-kit (CD117) and DOG-1 best identifies GIST.