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Neutrophil extracellular traps (NETs) are incriminated in several immune and inflammatory diseases including ulcerative colitis (UC). Analysis of colonic tissues for NETs-related markers in UC carries prognostic and therapeutic implications. This work aims to evaluate the immunohistochemical (IHC) expression of NETs-associated-protein arginine deaminase 4 (PAD4) in colonic biopsies from UC patients in comparison to normal colon (NC). Association between PAD4 expression level and histopathologic grade, patient's therapeutic response and other clinicopathological prognostic predictors in UC are determined. This cohort study included biopsies from 42 UC patients and 11 NC controls. Clinicopathological data including patient's age at diagnosis, gender, presenting symptoms, anatomical disease extent, extra-intestinal manifestations, type and response to therapy and surgical interventions were recorded and tabulated. Histopathological grading of disease activity and associated epithelial changes were assessed. PAD4 immunostaining was conducted using Horseradish Peroxidase technique and scored semiquantitatively considering intensity and percentage of nuclear staining of lamina propria inflammatory cells. Appropriate statistical tests were applied. Anti-PAD4 was localized mainly in the nuclei of lamina propria infiltrating neutrophils. It was expressed more significantly in UC (95.2 %) compared to NC (p 0.001). Increased PAD4 expression level was significantly associated with increasing histopathologic grade, anatomical disease extent, lacking response to therapy and subjection to radical surgery (p:0.001, = 0.038, 0.046, 0.046 respectively). Age, gender, presenting symptoms, extra-intestinal manifestations and epithelial changes showed insignificant associations. This study characterizes a subset of UC patients with high histopathological grade of activity, pancolonic involvement, strong/moderate PAD4 expression levels and who are unresponsive to routine medical therapeutic regimens rendering them candidates for radical surgery. In conjunction with histopathological grading, IHC evaluation of PAD4 in UC is recommended to guide patient's selection for targeted therapy using the novel-discovered selective PAD4 inhibitors. [ABSTRACT FROM AUTHOR]