In mammalian cells there are at least two Hsp90 isoforms, Hsp90a and Hsp90b which are encoded by separate genes. These ubiquitous and highly conserved proteins account for 1-2% of all cellular proteins in most cells. Hsp90 is part of the cells powerful network of chaperones to fight the deleterious consequences of protein unfolding caused by nonphysiological conditions. However, in the absence of stress, Hsp90 is a necessary component of fundamental cellular processes such as hormone signaling and cell cycle control. In this context several key regulatory proteins such as steriod receptors, cell cycle kinases involved in signal transduction and p53 have been identified as substrates of Hsp90. It has been suggested that Hsp90 acts as a capacitor for morphological evolution by buffering widespread variation, which may affect morphogenic pathways.