Abstract Purpose: Carbohydrate antigen 19-9 (CA19-9) has limited utility in the ∼10% of patients with pancreatic ductal adenocarcinoma (PDAC) who are Lewis-negative CA19-9 “nonproducers” (FUT3-null). This study aims to define the prognosis of FUT3-null PDAC and establish a CA19-9 cutoff that can help identify this subgroup in clinical practice. Experimental Design: Germline whole-exome sequencing was performed in a multicenter cohort of 615 patients with PDAC to determine FUT2/FUT3 genotypes. Re