Overview:

The HA epitope tag (YPYDVPDYA) consists of 9 amino acid residues and comes from the Human influenza virus hemagglutinin [1]. Hemagglutinin is a viral cell surface glycoprotein responsible for the binding of viral particles to their target cell, and is named because of its ability to agglutinate erythrocytes in vitro [2].

References:

1. Field, J. et al: Purification of a RAS-responsive adenylyl cyclase complex from Saccharomyces cerevisiae by use of an epitope addition method. Mol. Cell. 1988 Biol. 8: 2159-2165

2. Medeiros, R, et al., Hemagglutinin residues of recent human A(H3N2) influenza viruses that contribute to the inability to agglutinate chicken erythrocytes. Virology. 2001; Oct. 10; 289(1): 74–85