Angiotensin II type 1 receptor (181-187) is a 9-amino acid sequence of the second extracellular loop of angiotensin II (Ang II) in the renin-angiotensin system. Angiotensin II type 1 receptor (181-187) serves as the localization epitope of angiotensin II receptor autoantibodies (AT1RaAb) and is used to detect and quantify AT1RaAb levels in serum.