| Summary: | Here, we report a very rare case of melanocytic nevus of the colon. A 54-year-old woman with an unremarkable medical history visited our hospital for screening colonoscopy. Colonoscopy revealed a pigmented flat lesion in the ascending colon. Histological evaluation of a biopsy specimen revealed proliferation of pigmented cells in the lamina muscularis propria. On immunohistochemical analysis, pigmented cells were positive for S-100 protein and Melan-A expression, and a diagnosis of colonic melanocytic nevus was made. Although the pathogenesis underlying colonic melanocytic nevi remains unclear, we speculate that colonic melanocytic nevi develop via abnormal migration or differentiation of neural crest cells during embryogenesis.
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