The 90 kDa heat shock protein (hsp90) family of molecular chaperones is a highly conserved family of proteins that play an important physiological role. Hsp90 is involved in numerous cellular processes but is best known for its association with signal transduction machinery. A recently cloned homolog of hsp90 is the tumor necrosis factor receptor-associated protein (TRAP1). Like hsp90, TRAP1 is found to beassociated with numerous proteins involved in diverse actions (1, 2). Immunofluorescence data has shown TRAP1 to be localized in the mitochondria of mammalian cells. This observation and the fact that TRAP1 is shown to have a mitochondrial targeting presequence strongly implicates TRAP1 as a mitochondrial matrix protein (3).