Biliary stricture is mainly malignant in the adults and caused by several types of fatal malignancies such as pancreatic cancer, cholangiocarcinoma, and metastatic tumor, which have poor prognosis that the overall survival of unresectable lesions is no more than 15 months. The poor outcome often relates to a lack of reliable strategies for early diagnosis, which results in most patients with malignant biliary stricture being already advanced-stage disease at presentation. Therefore, it is critical to discover novel and effective strategies for the early diagnosis of malignant biliary strictures. Brush cytology and biopsy during endoscopic retrograde cholangiopancreatography (ERCP) are the main methods for recognizing malignant diseases of the bile duct, but their sensitivity is relatively low, 45% and 48.1%, respectively. Even when combined with other biomarkers like carbohydrate antigen 19-9 (CA19-9), their sensitivity is still less than 80%. In the previous study, the investigators found that bcf-eccDNA has excellent diagnostic value in predicting uncertain bile duct stricture, and the sensitivity and specificity of a related eccDNA in 40 samples are 80.8% and 100%. The sensitivity and specificity of another eccDNA were 92.3% and 92.9%, respectively. However, the sample size is still relatively small, and further prospective studies are needed to evaluate its diagnostic efficacy.