Figure 4: Three cases of interests in the study. a. A case of KIT-negative GIST (Case 27), immunohistochemically positive for PDGFRA and negative for c-KIT and other markers (×1000 magnification). Sequencing after subcloning of a double band sample on PCR electrophoresis of KIT exon 11 revealed a common deletion KVV (558-560). b. A case of GIST with simultaneous mutations of KIT and PDGFRA genes (Case 1), immunohistochemically-positive for c-KIT and negative for PDGFRA (×1000 magnification). 1. Sequencing after subcloning of a double band sample on PCR electrophoresis of KIT exon 11 revealed a common deletion KVV (558-560). 2. Direct sequencing of a single band sample on PCR electrophoresis of PDGFRA exon 18 revealed a point mutation S847L. 3. The point mutation of PDGFRA exon18 was confirmed after using the subcloning approach in this case of interest. c. Pre-and-post imatinib lesions obtained from a GIST patient (Case 28). 1. Pre-imatinib specimen presented fascicular arranged high cellular spindle cell morphology and immunohistochemically positive for c-KIT and DOG1 proteins and negative for desmin (×1000 magnification). Sequencing after subcloning of a single band sample on PCR electrophoresis of KIT exon 11 revealed a common deletion KVV (558-560). 2. Post-imatinib specimen presented spindle and epithelioid cell morphology and immunohistochemically showed complete loss of previous positivity for c-KIT and DOG1 proteins and diffuse strong positive staining for desmin (×1000 magnification). Sequencing after subcloning of a double band sample on PCR electrophoresis of KIT exon 11 revealed mutation of Q556H, WKVVEEI (557-563).