Previously, we identified anti-CaSR antibodies in patients with APS1 using immunoprecipitation assays.(16) The aim of this study was to define the epitope specificity of patient anti-CaSR antibodies using peptide phage display, a method that has been employed successfully in identifying autoantigenic epitopes in several autoimmune diseases.(25–27). Here, CASR is linked to Autoimmune polyendocrinopathy type 1.