The heat shock protein-90 kDa (HSP90) is a composite name for a large group of genes whose molecular weights average 90 kDa. HSP90 functions primarily as a molecular chaperone, facilitating the folding of other cellular proteins, preventing protein aggregation, or targeting improperly folded proteins to specific degradative pathways. In addition to its role as a molecular chaperone and stress response protein, HSP90 is a central component in a number of cellular processes including hormone signaling and cell cycle control. HSP90 is ubiquitously expressed, highly conserved, and accounts for 1-2% of the total cellular protein. Eukaryotic cells have two principal forms of HSP90 including the inducible HSP90α (also known as HSP90AA1, HSP90A, HSPC1, HSPCA and HSP86) and the constitutively expressed HSP90β (also known as HSP90AB1, HSP90B, HSPCB, HSPC2, and HSP89-β. While HSP90α and HSP90β share 90% identity, this affinity-purified polyclonal antibody is specific for HSP90α.