supply problems associated with GA derivatives and radicicol, together with GA toxicity problems, led Chiosis et al. to propose use of simple substituted adenine derivative as a potential base molecule. Significantly, the proposal was based on considering which praticular substructures might provide ATP-mimics with improved binding characteristics, rather than on computerized modeling. Thus, knowledge of the requirements of the ATP-binding pocket of Hsp90,and demonstration that a small molecule could function as a cytostatic agent, provide the intellectual stimulus for designing the ppurine-based PU class of compounds.Rational changes in the substituents in both rings and alteration of the length and rigidity of the linker gave rise to PU24FCL,which, although not the most active in the series,was utilized to further investigate Hsp90 inhibition in both healthy and tumor cells. The extensive effects exhibited by both healthy and tumor tissues when exposed to the compound have been reported, and,as with 17-AAG and GA, PU24FCL exhibited at least 10 to 50-fold lower affinity for Hsp90s from healthy tissues as compared with those from transformed cells.