(A) Analysis of apoptosis (TUNEL), cell proliferation (PCNA), and microvessel density (CD31). Mice were treated with control, gemcitabine, AEE 788, STI571, or the combination of AEE 788 and gemcitabine, STI571 and gemcitabine, AEE788 and STI571, or AEE788 and STI571 and gemcitabine. Pancreatic tumors were resected and processed for immunohistochemical evaluation of PCNA, TUNEL, and CD31 as described in Materials and Methods. (B) Double immunofluorescence staining of CD31/PECAM-1 and TUNEL in pancreatic tumors from mice treated with the combination of AEE788, STI571, and gemcitabine. Endothelial cells (CD31+) stained red fluorescence and apoptotic cells (TUNEL+) stained green fluorescence. Colocalization of endothelial cells undergoing apoptosis yielded yellow fluorescence.