The Er blood group system consists of five antigens carried on Piezo Type Mechanosensitive Ion Channel Component 1 (Piezo1). The protein comprises 2521 amino acid residues, 36 transmembrane domains, and one large extracellular domain (amino acids 2198 to 2431). Piezo1 is an important non-selective cation channel in a variety of tissues. The protein is encoded by a highly polymorphic single gene PIEZO1, associated with pathogenic mutations (e.g. dehydrated hereditary stomatocytosis, generalized lymphatic dysplasia, hemolytic anemia).