The purpose of this prospective study is to compare the diagnostic utility of two techniques (brush cytology + FISH and brush cytology + free DNA analysis) in the diagnosis of biliary strictures. Histologic diagnosis (biopsies) in conjunction with clinical and/or imaging follow-up will serve as the gold standard for diagnosis of malignancy. In order to do this the investigators will ask study participants to have a small volume of fluid obtained from the bile duct sent for additional testing at RedPATH. In some patients additional brushings will be obtained for FISH testing (this adds <2 minutes to ERCP and only associated risk is increased procedure duration).