STAT3 and gastric neoplasm: ,21,22,23,29 Indeed, using the nuclear presence of the phosphorylated STAT3 (pSTAT3) isoform as a surrogate marker, we identified active STAT3 signaling in epithelial cells localized in the mucosa as well as the submucosal invasive fronts across gastric tumors of KPP and KPT mice (Figure 3A), while in unaffected normal antrum, only a proportion of the epithelial cells stained positive for pSTAT3 (Figure S4A).