The HA tag sequence is derived from influenza hemagglutinin and is typically used to facilitate the detection of proteins. Tags such as HA used in conjunction with proteins can aid in purification, identification, and functional analysis of the tagged protein.Through the use of standard molecular biology techniques, a cDNA, which encodes the protein of interest can be cloned, in- frame, into an expression vector containing the tag sequence. The resultant tagged protein is usually referred to as an epitope-tagged protein or a fusion protein. Currently, a variety of expression vectors containing tags are commercially available. Depending on the expression vector selected, these fusion proteins may be expressed in a variety of organisms including bacteria, yeast, insect cells, and mammalian cells. Detection of a tagged protein is facilitated by the use of an antibody directed against the particular tag. This alleviates the need to generate a specific antibody against the protein itself. Therefore, newly identified proteins can be expressed and studied without having to wait for a specific antibody to be generated. Some of the more commonly used fusion tags include: c-myc, FLAG, GFP, GST, HA, His, and MBP.