Hsp90 (also known as Hsp82, Hsp83, or Hsp89), a 90 kDa protein, is the most abundant protein in the cytosolic fraction in many cell types. The high level of Hsp90 in many cell types sμggests that Hsp90 may play a general role in the cell, but little is known about its general function. Hsp90 has no known enzymatic activity, and thus it has been presumed that Hsp90 may function throμgh protein-protein interactions. Hsp90 exists in a dimeric form and has been observed to bind to several cellular proteins such as retro-virus kinases (PP60v-src), steroid receptors, heme-regulated protein kinase, actin and tubulin. In this regard, Hsp90, like Hsp70, may function as a “molecular chaperone”. Hsp90 exists as two isoforms referred to as Hsp86 and HSP84 in murine cells or Hsp90α and Hsp90β in human cells.