Hsp104 is a molecular chaperone required for stress tolerance and for maintenance of [psi(+)] prions in the budding yeast Saccharomyces cerevisiae (1,2). HSP104 is a hexameric protein with two AAA ATPase domains (N- and C-terminal nucleotide-binding domains NBD1 and NBD2, respectively) per monomer (3). NBD1 and NBD2 have very different catalytic properties, but each shows positive cooperativity in hydrolysis (3,4). Point mutations in either of the two nucleotide-binding domains (NBD) of Hsp104 (NBD1 and NBD2) eliminate its thermotolerance function in vivo (5). Hsp104 interacts with Hsp90 cochaperones in respiring yeast (1). The primary function of Hsp104 in prion propagation is to disassemble prion aggregates and generate the small prion seeds that initiate new rounds of prion propagation (possibly assisted by Hsp70-Ssa) (6).