A 52-year-old man underwent screening colonoscopy. A sessile serrated lesion (Paris 0-Is, NICE-1; 17 mm diameter) was observed in the right colon. After submucosal injection with gelofusine plus indigo carmine, en bloc cold endoscopic mucosal resection (cold-EMR) was attempted using a 25-mm braided snare (SnareMaster SD-210U-25; Olympus, Tokyo, Japan) (▶ Fig. 1). After snare closure, moderate traction of the snare sheath was applied to remove the lesion; however, the lesion remained trapped due to a large amount of tissue caught. The snare was repositioned slightly, and resection was successful. On inspection of the mucosal defect, the submucosal layer was observed with apparent "bubble sign" after waterjet irrigation (▶ Fig. 2 a, ▶ Video 1), followed by a "double string" sign, suggesting exposure of damaged muscle fibers (▶ Fig. 2 b). After insufflation and water irrigation, these muscle fibers separated showing deep muscular injury with visible serosa (▶ Fig. 2 c). The defect was immediately closed with endoscopic clips (▶ Fig. 2 d). The patient was monitored for 4 hours and discharged with prophylactic antibiotics and no symptoms. No complications occurred. Histology confirmed a 21 × 12 × 2 mm specimen and suspicion of serrated lesion without dysplasia. E-Videos Video 1 The "double string" sign and perforation after cold endoscopic mucosal resection. ▶ Fig. 1 Sessile serrated lesion Paris 0-Is, NICE-1, in the right colon. ▶ Fig. 2 The mucosal defect after cold endoscopic mucosal resection. a Submucosal layer with apparent "bubble sign" after waterjet irrigation. b Injured muscle layer, "double string" image. c Perforation site. d Complete endoscopic closure with clips. Nogales Óscar et al. The "double string" warning sign after cold EMR … Endoscopy

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