A multitude of heat shock transcription factors (HSFs) have been isolated and characterized from various plant species (17-23). Based on a phylogeny analysis of the DNA binding domains and organization of oligomerization domains, they have been assigned to class A and B of the plant HSF family (20,24 and this paper). None of the tested soybean or Arabidopsis HSF class B members were able to function as transcriptional activators and are, therefore, considered to be inert (26,59). Conversely, class A HSFs from tomato and Arabidopsis displayed an intrinsic transcriptional activation potential (26,50). There seems to be variation among plant class A HSFs regarding their transcriptional activation functions: some play a key role in activation of the heat shock response, while others act in an auxiliary capacity as HSF activity boosters (54). In contrast, the class B inert HSFs are able to trans-attenuate the transcriptional activity of activator HSFs (26). We postulated that heat shock regulation in plants may differ from metazoans by partitioning negative and positive functional domains onto separate HSF proteins (59). In plants two classes of HSFs exist: class A members which function as activators of HSP gene expression, and a novel class B (inert HSFs) which is largely specialized for repression, or attenuation, of the heat shock response.